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托福考试练习试题2017年
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Groundwater is the word used to describe water that saturates the ground, filling all theavailable spaces. By far the most abundant type of groundwater is meteoric water; this isthe groundwater that circulates as part of the water cycle. Ordinary meteoric water is waterthat has soaked into the ground from the surface, from precipitation (rain and snow) andfrom lakes and streams. There it remains, sometimes for long periods, before emerging at thesurface again. At first thought it seems incredible that there can be enough space in the“solid” ground underfoot to hold all this water.
The necessary space is there, however, in many forms. The commonest spaces are thoseamong the particles—sand grains and tiny pebbles—of loose, unconsolidated sand and gravel.Beds of this material, out of sight beneath the soil, are common. They are found wherever fastrivers carrying loads of coarse sediment once flowed. For example, as the great ice sheets thatcovered North America during the last ice age steadily melted away, huge volumes of waterflowed from them. The water was always laden with pebbles, gravel, and sand, known asglacial outwash, that was deposited as the flow slowed down.
The same thing happens to this day, though on a smaller scale, wherever a sediment-ladenriver or stream emerges from a mountain valley onto relatively flat land, dropping its load asthe current slows: the water usually spreads out fanwise, depositing the sediment in the formof a smooth, fan-shaped slope. Sediments are also dropped where a river slows on entering alake or the sea, the deposited sediments are on a lake floor or the seafloor at first, but will belocated inland at some future date, when the sea level falls or the land rises; such beds aresometimes thousands of meters thick.
In lowland country almost any spot on the ground may overlie what was once the bed of a riverthat has since become buried by soil; if they are now below the water’s upper surface (thewater table), the gravels and sands of the former riverbed, and its sandbars, will be saturatedwith groundwater.
So much for unconsolidated sediments. Consolidated (or cemented) sediments, too, containmillions of minute water-holding pores. This is because the gaps among the original grains areoften not totally plugged with cementing chemicals; also, parts of the original grains maybecome dissolved by percolating groundwater, either while consolidation is taking place or atany time afterwards. The result is that sandstone, for example, can be as porous as the loosesand from which it was formed.
Thus a proportion of the total volume of any sediment, loose or cemented, consists of emptyspace. Most crystalline rocks are much more solid; a common exception is basalt, a form ofsolidified volcanic lava, which is sometimes full of tiny bubbles that make it very porous.
The proportion of empty space in a rock is known as its porosity. But note that porosity isnot the same as permeability, which measures the ease with which water can flow through amaterial; this depends on the sizes of the individual cavities and the crevices linking them.
Much of the water in a sample of water-saturated sediment or rock will drain from it if thesample is put in a suitable dry place. But some will remain, clinging to all solid surfaces. It isheld there by the force of surface tension without which water would drain instantly from anywet surface, leaving it totally dry. The total volume of water in the saturated sample musttherefore be thought of as consisting of water that can, and water that cannot, drain away.
The relative amount of these two kinds of water varies greatly from one kind of rock orsediment to another, even though their porosities may be the same. What happens dependson pore size. If the pores are large, the water in them will exist as drops too heavy for surfacetension to hold, and it will drain away; but if the pores are small enough, the water in them willexist as thin films, too light to overcome the force of surface tension holding them in place;then the water will be firmly held.
Paragraph 1: Groundwater is the word used to describe water that saturates the ground, fillingall the available spaces. By far the most abundant type of groundwater is meteoric water;this is the groundwater that circulates as part of the water cycle. Ordinary meteoric water iswater that has soaked into the ground from the surface, from precipitation (rain and snow)and from lakes and streams. There it remains, sometimes for long periods, before emerging atthe surface again. At first thought it seems incredible that there can be enough space in the“solid” ground underfoot to hold all this water.
1. Which of the following can be inferred from paragraph 1 about the ground that we walk on?
○It cannot hold rainwater for long periods of time.
○It prevents most groundwater from circulating.
○It has the capacity to store large amounts of water.
○It absorbs most of the water it contains from rivers.
2. The word “incredible” in the passage is closest in meaning to
○Confusing
○Comforting
○Unbelievable
○Interesting
Paragraph 2: The necessary space is there, however, in many forms. The commonest spacesare those among the particles—sand grains and tiny pebbles—of loose, unconsolidated sandand gravel. Beds of this material, out of sight beneath the soil, are common. They are foundwherever fast rivers carrying loads of coarse sediment once flowed. For example, as the greatice sheets that covered North America during the last ice age steadily melted away, hugevolumes of water flowed from them. The water was always laden with pebbles, gravel, andsand, known as glacial outwash, that was deposited as the flow slowed down.
3. The word “out of sight” in the passage is closest in meaning to
○Far away
○Hidden
○Partly visible
○Discovered
4. According to paragraph 2, where is groundwater usually found?
○Inside pieces of sand and gravel
○On top of beds of rock
○In fast rivers that are flowing beneath the soil
○In spaces between pieces of sediment
5. The phrase “glacial outwash” in the passage refers to
○Fast rivers
○Glaciers
○The huge volumes of water created by glacial melting
○The particles carried in water from melting glaciers.
Paragraph 3: The same thing happens to this day, though on a smaller scale, wherever asediment-laden river or stream emerges from a mountain valley onto relatively flat land,dropping its load as the current slows: the water usually spreads out fanwise, depositing thesediment in the form of a smooth, fan-shaped slope. Sediments are also dropped where ariver slows on entering a lake or the sea, the deposited sediments are on a lake floor or theseafloor at first, but will be located inland at some future date, when the sea level falls or theland rises; such beds are sometimes thousands of meters thick.
6. All of the following are mentioned in paragraph 3 as places that sediment-laden rivers candeposit their sediments EXCEPT
○A mountain valley
○Flat land
○A lake floor
○The seafloor
Paragraph 4: In lowland country almost any spot on the ground may overlie what was once thebed of a river that has since become buried by soil; if they are now below the water’s uppersurface (the water table), the gravels and sands of the former riverbed, and its sandbars, will besaturated with groundwater.
7. The word “overlie” in the passage is closest in meaning to
○Cover
○Change
○Separate
○Surround
Paragraph 5: So much for unconsolidated sediments. Consolidated (or cemented) sediments,too, contain millions of minute water-holding pores. This is because the gaps among theoriginal grains are often not totally plugged with cementing chemicals; also, parts of theoriginal grains may become dissolved by percolating groundwater, either while consolidationis taking place or at any time afterwards. The result is that sandstone, for example, can be asporous as the loose sand from which it was formed.
8. The phrase “so much for” in the passage is closest in meaning to
○That is enough about
○Now let us turn to
○Of greater concern are
○This is related to
9. The word “plugged” in the passage is closet in meaning to
○Washed
○Dragged
○Filled up
○Soaked through
Paragraph 6: Thus a proportion of the total volume of any sediment, loose or cemented,consists of empty space. Most crystalline rocks are much more solid; a common exception isbasalt, a form of solidified volcanic lava, which is sometimes full of tiny bubbles that make itvery porous.
Paragraph 7: The proportion of empty space in a rock is known as its porosity. But note thatporosity is not the same as permeability, which measures the ease with which water can flowthrough a material; this depends on the sizes of the individual cavities and the crevices linkingthem.
10. According to paragraphs 6 and 7, why is basalt unlike most crystalline forms of rock?
○It is unusually solid
○It often has high porosity.
○It has a low proportion of empty space.
○It is highly permeable.
11. What is the main purpose of paragraph 7?
○To explain why water can flow through rock
○To emphasize the large amount of empty space in all rock
○To point out that a rock cannot be both porous and permeable
○To distinguish between two related properties of rock
Paragraph 9: The relative amount of these two kinds of water varies greatly from one kind ofrock or sediment to another, even though their porosities may be the same. What happensdepends on pore size. If the pores are large, the water in them will exist as drops too heavy forsurface tension to hold, and it will drain away; but if the pores are small enough, the water inthem will exist as thin films, too light to overcome the force of surface tension holding them inplace; then the water will be firmly held.
12. Which of the sentences below best expresses the essential information in the highlightedsentence in the passage? Incorrect choices change the meaning in important ways or leave outessential information.
○Surface tension is not strong enough to retain drops of water in rocks with large pores butit strong enough to hold on to thin films of water in rocks with small pores.
○Water in rocks is held in place by large pores and drains away from small size pores throughsurface tension.
○Small pores and large pores both interact with surface tension to determine whether a rockwill hold water as heavy drops or as a thin film.
○If the force of surface tension is too weak to hold water in place as heavy drops, the waterwill continue to be held firmly in place as a thin film when large pores exist.
Paragraph 8: Much of the water in a sample of water-saturated sediment or rock will drainfrom it if the sample is put in a suitable dry place.█ But some will remain, clinging to all solidsurfaces.█ It is held there by the force of surface tension without which water would draininstantly from any wet surface, leaving it totally dry.█ The total volume of water in thesaturated sample must therefore be thought of as consisting of water that can, and waterthat cannot, drain away.█
13. Look at the four squares [█] that indicate where the following sentence could be added tothe passage.
What, then, determines what proportion of the water stays and what proportion drainsaway?
Where would the sentence best fit? Click on a square to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the mostimportant ideas in the passage. Some sentences do not belong in the summary because theyexpress ideas that are not presented in the passage or are minor ideas in the passage. Thisquestion is worth 2 points.
Much of the ground is actually saturated with water.
Answer choices
○Sediments that hold water were spread by glaciers and are still spread by rivers andstreams.
○Water is stored underground in beds of loose sand and gravel or in cemented sediment.
○The size of a saturated rock’s pores determines how much water it will retain when the rockis put in a dry place.
○Groundwater often remains underground for a long time before it emerges again.
○Like sandstone, basalt is a crystalline rock that is very porous.
○Beds of unconsolidated sediments are typically located at inland sites that were onceunderwater.
1. ○ 3
2. ○ 3
3. ○ 2
4. ○ 4
5. ○ 4
6. ○ 1
7. ○ 1
8. ○ 1
9. ○ 3
10. ○ 2
11. ○ 4
12. ○ 1
13. ○ 4
14. ○1 2 3
参考译文：
地下水
地下水是指渗入到地下并将所有岩石空隙填满的水。到现在为止，雨水是最丰富的地下水资源，是地下水在水循环中的一个环节。普通的雨水会从地表、降水以及湖泊河流侵入到地下。在冒出地表之前，这些地下水有时会长时间留在地下。在我们脚下坚实的土地中竟然有足够的空间储存这些水，这在一开始会让人觉得难以置信。
地下水所需的储存空间多种多样。松散的砂子和砾石间有许多颗粒和小石子，他们之间的空隙是最常见的储存地下水的空间。由这些颗粒组成的水床非常普遍，通常位于看不见的土壤下方，在湍急的河流曾经流过的地方都能找到它们的踪迹。比如，冰河时代覆盖北美的巨大冰层逐渐融化，大量水从那儿流出。水里总会携带些石子、砾石和沙石，这就是所谓的冰河期的冰水沉积，这些颗粒会随着水流的减速而沉淀。
现代也有冰水沉积，尽管规模相对较小。凡是有携带泥沙的河流或者溪流从山谷流至相对平坦的地面时，砂石就随着水流速度的减慢逐渐沉淀;水流通常呈扇形扩散，它们所携带的砂石也会沉淀为光滑的扇形斜面。当河流汇入湖泊和海洋的时候也会有沉淀，这些沉淀最初在湖底或海底，但将来海平面下降或者陆地崛起时，它们就会分布于内陆，通常厚达几千米。
低地区域上的任何位置可能就是曾经的河床，后续被土壤覆盖而变成现在的样子。如果那些河床和沙洲现在位于地下水位之下，一定会有大量的地下水浸在它们的沙砾和沙石之间。
以上说的都是松散的沉积物，那些坚固的沉积物，也拥有以数万计的毛细孔来容纳水。因为最初颗粒间的缝隙通常并未完全被粘固的化学物质塞满，而且部分颗粒很可能在固化时或固化后被渗入的地下水溶解;结果这些砂岩最终变得和形成它的散沙一样多孔。
因此，不管沉积物是疏松还是坚固，他们中一定有空间。大部分结晶体岩石都非常坚硬，但也有例外，最常见的就是玄武岩，它是一种固化的火山熔岩，经常充满了微小气泡，从而变得十分多孔。
岩石的多孔性就是指其中空隙的比例。但需要注意的是，多孔性与渗透性是不同的。渗透性衡量的是水渗透物质的难易程度，它取决于与单个空隙以及连接空隙间裂缝的大小。
当充满水分的沉淀物或者岩石样本被放置在适合的干燥环境中时，大部分的水分会流干，但仍有部分水会继续附着在坚实的表面上。要不是因为表面张力，这些水分也会立刻蒸发，仅留下完全干燥的样本。因此，试验样本的含水量既包括可以流干的水，也包括不能流干的水。
这两种水的相对含量因岩石或沉积物种类不同而改变，即便它们有相同比例的空隙，还取决于空隙的大小。如果空隙很大，其中的水会形成水滴，太重足以克服吸引它的表面张力，就会流走;但如果空隙够小，水会像薄膜一样，太轻无法克服表面张力，从而稳稳地附着在空隙表面上。
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