Groundwateris the word used todescribewater that saturates the ground, filling all theavailablespaces. By far the mostabundanttype ofgroundwaterismeteoricwater; this is thegroundwaterthat circulates as part of the watercycle. Ordinarymeteoricwater is water that has soaked into the ground from the surface, fromprecipitation(rain and snow) and from lakes and streams. There it remains, sometimes for long periods, before emerging at the surface again. At first thought it seemsincrediblethat there can be enoughspacein the “solid” ground underfoot to hold all this water.

The necessaryspaceis there, however, in many forms. The commonest spaces are those among the particles—sand grains and tiny pebbles—ofloose,unconsolidatedsand andgravel. Beds of thismaterial, out of sightbeneaththe soil, arecommon. They are found wherever fast rivers carrying loads ofcoarsesedimentonce flowed. For example, as the great ice sheets that covered North America during the last ice age steadily melted away, huge volumes of water flowed from them. The water was always laden with pebbles,gravel, and sand, known asglacialoutwash, that was deposited as the flow slowed down.

The same thing happens to this day, though on a smallerscale, wherever a sediment-laden river orstreamemerges from a mountainvalleyontorelativelyflat land, dropping itsloadas thecurrentslows: the water usually spreads out fanwise, depositing thesedimentin the form of a smooth, fan-shapedslope. Sediments are also dropped where a river slows on entering a lake or the sea, the deposited sediments are on a lake floor or the seafloor at first, but will be located inland at some futuredate, when the sealevelfalls or the land rises; such beds are sometimes thousands of meters thick.

Inlowlandcountry almost anyspoton the ground may overlie what was once the bed of a river that has since become buried by soil; if they are now below the water’s upper surface (the water table), the gravels and sands of theformerriverbed, and its sandbars, will be saturated withgroundwater.

So much forunconsolidatedsediments. Consolidated (or cemented) sediments, too,containmillions ofminutewater-holding pores. This is because the gaps among theoriginalgrains are often not totally plugged with cementing chemicals; also, parts of theoriginalgrains may become dissolved by percolatinggroundwater, either while consolidation is taking place or at any time afterwards. The result is that sandstone, for example, can be asporousas theloosesand from which it was formed.

Thus aproportionof the totalvolumeof anysediment,looseor cemented, consists of emptyspace. Mostcrystallinerocks are much moresolid; acommonexceptionis basalt, a form of solidifiedvolcaniclava, which is sometimes full of tiny bubbles that make it veryporous.

Theproportionof emptyspacein a rock is known as itsporosity. But note thatporosityis not the same as permeability, which measures theeasewith which water can flowthroughamaterial; this depends on the sizes of theindividualcavities and the crevices linking them.

Much of the water in asampleof water-saturatedsedimentor rock willdrainfrom it if thesampleis put in asuitabledry place. But some willremain, clinging to allsolidsurfaces. It is held there by the force of surfacetensionwithout which water woulddraininstantlyfrom any wet surface, leaving it totally dry. The totalvolumeof water in the saturatedsamplemustthereforebe thought of as consisting of water that can, and water that cannot,drainaway.

Therelativeamountof these two kinds of water varies greatly from one kind of rock orsedimentto another,eventhough their porosities may be the same. What happens depends onporesize. If the pores are large, the water in them will exist as drops too heavy for surfacetensionto hold, and it willdrainaway; but if the pores are small enough, the water in them will exist as thin films, too light toovercomethe force of surfacetensionholding them in place; then the water will be firmly held.

Questions:
question_stem: Which of the following can be inferred from paragraph 1 about the ground that we walk on?
choices: tag: A
text: It cannot hold rainwater for long periods of time.

tag: B
text: It prevents most groundwater from circulating.

tag: C
text: It has the capacity to store large amounts of water.

tag: D
text: It absorbs most of the water it contains from rivers.
answer_key: C
explanation: 以下哪一项能够从第一段推出与我们走的路地有关？（Inference Question）A 它不能长时间储存水。 

B 它阻止了大部分地下水进行水循环。 

C 它有能力储存大量的地下水。 

D 它大部分的水从河流中吸收。根据“ground that we walk on”定位到段落最后一句话“At first thought it seemsincrediblethatthere can be enough spacein the “solid” ground underfootto hold all this water.”意思是：一开始很难想象我们脚下的地有足够的空间容下这些水。
 
选项 A 错，倒数第二句“There it remains, sometimesfor long periods, before emerging at the surface again.”For long periods 长时间；remain v.保持；emerge v.出现；
 
选项 B 错，段落第二句“By far themost abundanttype of groundwater is meteoric water; this is the groundwater that circulates as part of thewater cycle.”意思是：现在最丰富的地下水类型是雨水；这些雨水是进入水循环的那部分；by far 最；abundant adj.富足；circulate v.循环；
 
选项 D 错，段落第三句“Ordinary meteoric water is water that has soaked into the groundfrom the surface,from precipitationandfrom lakes and streams.”意思是：平常的雨水都是那些从地表、降水和河流湖泊中渗入地下的水。soak v.渗透；precipitationn.降雨，降雪；

question_stem: The word “incredible” in the passage is closest in meaning to
choices: tag: A
text: confusing

tag: B
text: comforting

tag: C
text: unbelievable

tag: D
text: interesting
answer_key: C
explanation: "incredible: 令人难以置信的。对应 C 选项。A 是令人困惑的；B 是令人宽慰的；D 是有意思的。对应原文 At first thought it seems incredible that there can be enough space in the "solid" ground underfoot to hold all this water.想到 credit card 信用卡，credit 指的是信用或者学分 ible 或者 able 表示可以……的，credible 可信的，incredible 难以置信的。从语境中也可以读出惊叹之意。

question_stem: The word “out of sight” in the passage is closest in meaning to
choices: tag: A
text: far away

tag: B
text: hidden

tag: C
text: partly visible

tag: D
text: discovered
answer_key: B
explanation: "out of sight: 看不见，在视野之外。A 是很遥远；B 是隐藏；C 是部分看的见；D 是发现。根据词意，B 最接近。对应原文 Beds of this material, out of sight beneath the soil, are common.表面意思就是在视野之外，也就是看不见， C 和 D 都说看见，所以错。而且 far away 离得很远也不代表就看不见，所以也不对。不知道的话说 beneath the soil 在土下面，当然也是看不见之意，所以 B 正确。"

question_stem: According to paragraph 2, where is groundwater usually found?
choices: tag: A
text: Inside pieces of sand and gravel

tag: B
text: On top of beds of rock

tag: C
text: In fast rivers that are flowing beneath the soil

tag: D
text: In spaces between pieces of sediment
answer_key: D
explanation: "问的是地下水在哪儿最经常在哪儿发现，找到第二句中的 the commonest spaces： The commonest spaces are those among the particles—sand grains and tiny pebbles—of loose, unconsolidated sand and gravel.最常见的地方是 blablabla，这个 blablabla 就是我们要的答案，罗列的是不同颗粒之间的空隙，所以答案 D 是正确的，答案把原文高度概括。"

question_stem: The phrase “glacial outwash” in the passage refers to
choices: tag: A
text: fast rivers

tag: B
text: glaciers

tag: C
text: the huge volumes of water created by glacial melting

tag: D
text: the particles carried in water from melting glaciers
answer_key: D
explanation: "glacial 是冰川的，与冰川有关的，outwash 是个合成词，表面意思就是冲出来的，不知道的读原文。原文说 pebble, gravel and sand, known as glacial outwash，也就是说前面的三个都叫做 glacial outwash，其中 sand 是一定知道的，不是水，所以 AC 错，更不是冰，B 错，应该是包括沙子在内的一堆小颗粒。"

question_stem: All of the following are mentioned in paragraph 3 as places that sediment-laden rivers can deposit their sediments EXCEPT
choices: tag: A
text: A mountain valley

tag: B
text: Flat land

tag: C
text: A lake floor

tag: D
text: The seafloor
answer_key: A
explanation: "可在第一句找到 B 的 flat land，B 正确，A 虽然有说到，但不是沉积物沉积的地点，从 from 可以看出，所以要选的答案是 A： The same thing happens to this day, though on a smaller scale, wherever a sediment-laden river or stream emerges from a mountain valley onto relatively flat land, dropping its load as the current slows.在第二句的前半句找到 C 和 D，CD 不正确： Sediments are also dropped where a river slows on entering a lake or the sea, the deposited sediments are on a lake floor or the seafloor at first..."

question_stem: The word “overlie” in the passage is closest in meaning to
choices: tag: A
text: cover

tag: B
text: change

tag: C
text: separate

tag: D
text: surround
answer_key: A
explanation: "overlie: 躺在…上面；覆盖在…上面。over 表在……上，lie 表躺，所以 overlie 的意思是躺在……上，也就是 A 选项的 cover.对应原文 In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil 原文说低地国家地上的任何一点都可能覆盖原来的河床，B 改变，C 分开，D 围绕全都不对。"

question_stem: The phrase “So much for” in the passage is closest in meaning to
choices: tag: A
text: that is enough about

tag: B
text: now let us turn to

tag: C
text: of greater concern are

tag: D
text: this is related to
answer_key: A
explanation: "so much for: 表面意思就是“已经很多了”，也就是足够的意思。A 是足够；B 是让我们转向；C 是极受关注；D 是这和…有关。根据词意，A 是正确选项。对应原文 So much for unconsolidated sediments. Consolidated (or cemented) sediments, too, contain millions of minute water-holding pores.代入原文，说 unconsolidated 未固结的沉积物已经说了很多了，下面就开始说固结的沉积物了，所以 A 正确，B 和 C 都说我们马上要讨论的是未固结的沉积物，所以说反了；D 表示与……相关，完全不沾边，错。"

question_stem: The word “plugged” in the passage is closet in meaning to
choices: tag: A
text: washed

tag: B
text: dragged

tag: C
text: filled up

tag: D
text: soaked through
answer_key: C
explanation: 与单词 plugged 在文中意思最接近的是（Vocabulary Question） 

a)Washed（冲刷） 

b)Dragged（拖，拉） 

c)Filled up（充满） 

d)Soaked through（渗入） 

解析：plug v.堵，赛，插上；含义与充满是一样的。

question_stem: According to paragraphs 6 and 7, why is basalt unlike most crystalline forms of rock?
choices: tag: A
text: It is unusually solid.

tag: B
text: It often has high porosity.

tag: C
text: It has a low proportion of empty space.

tag: D
text: It is highly permeable.
answer_key: B
explanation: "以 crystalline rock 和 basalt 为关键词定位至第六段的最后一句：Most crystalline rocks are much more solid; a common exception is basalt, a form of solidified volcanic lava, which is sometimes full of tiny bubbles that make it very porous.原文说大部分 crystalline rock 是固化比较好的，但 basalt 是个例外，说 basalt 更加 porous，孔隙率更高，所以 B 正确。"

question_stem: What is the main purpose of paragraph 7?
choices: tag: A
text: To explain why water can flow through rock

tag: B
text: To emphasize the large amount of empty space in all rock

tag: C
text: To point out that a rock cannot be both porous and permeable

tag: D
text: To distinguish between two related properties of rock
answer_key: D
explanation: "第七段整段说的都是有关岩石孔隙度和渗透率的问题。第七段的这两句中所表达的信息，我们可简化为 proportion 是 porosity，but porosity 不是 permeability。通过这两句作者区分了 porosity 和 permeability，并且这两句是在段首，说明了段落大意，和题干中所问的第七段主要目的相同。因此 D 选项正确"。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: Surface tension is not strong enough to retain drops of water in rocks with large pores but it strong enough to hold on to thin films of water in rocks with small pores.

tag: B
text: Water in rocks is held in place by large pores and drains away from small size pores through surface tension.

tag: C
text: Small pores and large pores both interact with surface tension to determine whether a rock will hold water as heavy drops or as a thin film.

tag: D
text: If the force of surface tension is too weak to hold water in place as heavy drops, the water will continue to be held firmly in place as a thin film when large pores exist.
answer_key: A
explanation: "原文的结构是如果孔隙大，就不容易 hold，如果孔隙小，就容易 hold。A 完全正确。选项 B 说反，应该是孔隙大的话水会流走；C 的 interact to determine 改变原句的意思和逻辑，错；D 选项在若干处都偷换了原文的概念，完全不对。"