Where does the water in a lake come from, and how does water leave it? Water enters a lake from inflowing rivers, from underwater seeps and springs, from overland flow off the surrounding land, and from rain falling directly on the lake surface. Water leaves a lake via outflowing rivers, by soaking into the bed of the lake, and byevaporation. So muchisobvious.

The questions become morecomplicatedwhen actual volumes of water are considered: how much water enters and leaves by eachroute? Discovering the inputs and outputs of rivers is a matter of measuring the discharges of every inflowing and outflowingstreamand river. Then exchanges with theatmosphereare calculated by finding thedifferencebetween the gains from rain, as measured (ratherroughly) by rain gauges, and the losses byevaporation, measured with models that correct for the other sources of water loss. For themajorityof lakes, certainly those surrounded by forests, input from overland flow is too small to have anoticeableeffect. Changes in lakelevelnot explained by river flows plus exchanges with theatmospheremust bedueto the netdifferencebetween what seeps into the lake from thegroundwaterand what leaks into thegroundwater. Note the word "net": measuring the actual amounts ofgroundwaterseepage into the lake and out of the lake is a much morecomplicatedmatter than merely inferring theirdifference.

Once all this information has been gathered, it becomes possible to judge whether a lake’s flow is mainlydueto its surface inputs and outputs or to itsundergroundinputs and outputs. If theformerare greater, the lake is a surface-water-dominated lake; if thelatter, it is a seepage-dominated lake. Occasionally,commonsensetells you which of these two possibilities applies. For example, a pond in hilly country that maintains asteadywaterlevelallthrougha dry summer in spite of having no streams flowing into it mustobviouslybe seepage dominated. Conversely, a pond with astreamflowing in one end and out the other, which dries up when thestreamdries up, is clearly surface water dominated.

By whatever means, a lake isconstantlygaining water and losing water: its water does not just sit there, or, anyway, not for long. This raises the matter of a lake’s residence time. The residence time is theaveragelength of time that anyparticularmoleculeof water remains in the lake, and it is calculated by dividing thevolumeof water in the lake by therateat which water leaves the lake. The residence time is anaverage; the time spent in the lake by agivenmolecule(if we could follow its fate) woulddependon therouteit took: it might flowthroughas part of the fastest, most directcurrent, or it might circle in a backwater for anindefinitelylong time.

Residence timesvaryenormously. They range from a few days for small lakes up to several hundred years for large ones; Lake Tahoe, in California, has a residence time of 700 years. The residence times for the Great Lakes of North America, namely, LakesSuperior, Michigan, Huron, Erie, and Ontario, are,respectively, 190,100,22,2.5, and 6 years. Lake Erie’s is the lowest: although its area is larger than Lake Ontario’ s, itsvolumeis less than one-third as great because it is so shallow-less than 20 meters onaverage.

Agivenlake's residence time is by no means a fixedquantity. It depends on therateat which water enters the lake, and that depends on therainfalland theevaporationrate. Climaticchange (the result of global warming?) isdramaticallyaffecting the residence times of some lakes in northwestern Ontario. Canada. In the period 1970 to 1986,rainfallin the area decreased from 1,000 millimeters to 650 millimeters per annum, while above-average temperatures speeded up the evapotranspirationrate(therateat which water is lost to theatmospherethroughevaporationand the processes of plant life).

The result has been that the residence time of one of the lakes increased from 5 to 18 years during the study period. The slowing down of waterrenewalleads to achainof further  consequences; it causes dissolved chemicals to become  increasingly concentrated, and this, in turn, has amarkedeffect on all living things in the lake.

Questions:
question_stem: The phrase "So much" in the passage refers to
choices: tag: A
text: the negative effects of overland flow, rain, and evaporation on river water levels

tag: B
text: water that a lake loses to outflowing rivers, to the lake bed, and to evaporation

tag: C
text: the importance of rivers to the maintenance of lake water levels

tag: D
text: the information given about ways that water can enter or exit a lake
answer_key: D
explanation: D So much 指代前文，前文说 water 是怎么 enter 怎么 leave 的，所以正确答案是 D。注意 B 貌似也对，但代入之后是有问题的，因为 water 不是 obvious 的，明显的是信息，因此 so much 不仅仅指代它的前句，还指代了前一句的前一句，正确答案是 D。

question_stem: The word "gains" in the passage is closest in meaning to
choices: tag: A
text: results

tag: B
text: increases

tag: C
text: resources

tag: D
text: savings
answer_key: B
explanation: B gain: 增加，获得。A 是结果，B 是增加，C 是资源，D 是储蓄，储存。根据词意，B 为正确答案。

question_stem: Which of the following can be inferred from paragraph 2 about the movement of water into a lake?
choices: tag: A
text: Heavy rain accounts for most of the water that enters into lakes.

tag: B
text: Rainfall replaces approximately the amount of water lost through evaporation.

tag: C
text: Overland flow into lakes is reduced by the presence of forests.

tag: D
text: Seepage has a smaller effect on water level than any other input.
answer_key: C
explanation: C 此题关键词难找，所以可以用排除法，当然也可以根据有序性往下找。这里我们用排除法。A 的 heavy rain 和 most of the water 原文完全没有相关信息，错.

question_stem: Why does the author use the phrase "Note the word 'net'" in the passage?
choices: tag: A
text: To emphasize the impact of seepage on water levels.

tag: B
text: To point out that seepage is calculated differently from river flows and atmospheric exchanges.

tag: C
text: To compare the different methods of calculating seepage.

tag: D
text: To emphasize the difficulty of obtaining specific values for seepage inputs and outputs.
answer_key: D
explanation: 该题对应本段最后一句，大意为：注意一下“净值”这个词：测量真正渗入和渗出湖中的地下水量比仅仅推断它们的不同要复杂的多。选项 D 说法正确，答案是 D。

question_stem: The word "Conversely" in paragraph 3 meaning to
choices: tag: A
text: on the other hand

tag: B
text: in the same way

tag: C
text: in other words

tag: D
text: on average
answer_key: A
explanation: A conversely: 相反地。A 选项是另一方面，B 是同样地，C 是换一句话说，D 是平均。所以正确答案是 on the other hand。

question_stem: According to paragraph 3, which of the following best describes a seepage-dominated lake?
choices: tag: A
text: A lake that is fed by streams but still has fluctuating water levels

tag: B
text: A lake with a constant water level that has no streams or rivers as inputs

tag: C
text: A lake with a stream flowing into it and a stream flowing out of it

tag: D
text: A lake that has surface and underground inputs but loses water during dry seasons
answer_key: B
explanation: B 以 seepage-dominated lake 做关键词定位至第二句或者倒数第二句，都是可以的。第二句说如果 underground 大的话就是 seepage，倒数第二句说如果没有地表水注入的 pond 依然能维持水量就是 seepage-dominated，所以无论定位至哪句话答案都是 B。其他三个答案都提到了地表水，都与原文相反，错。

question_stem: It can be inferred from paragraph 4 that the length of time a given molecule of water remains in a lake
choices: tag: A
text: depends entirely upon the average speed of a lake' s currents

tag: B
text: can be measured by the volume of the lake alone

tag: C
text: can be greater or lesser than the residence time

tag: D
text: is similar to the length of time all other molecules remain in that lake
answer_key: C
explanation: C  以 a given molecule of water 做关键词定位至最后一句，说 residence time 是平均值，一个分子在 lake 里的时间取决于它的 route，有可能很快也有可能很慢，所以正确答案是 C，或者多或者少。A 和 B 原文没说；D 说反，原文都说了一个具体的分子和 residence time 不一样，错。

question_stem: According to paragraph 5, Lake Erie's residence time is lower than Lake Ontario;s for which of the following reasons?
choices: tag: A
text: Lake Erie has a larger area than Lake Ontario.

tag: B
text: Lake Ontario is shallower than Lake Erie.

tag: C
text: Lake Ontario has a greater volume than Lake Erie.

tag: D
text: Lake Erie receives less rainfall than Lake Ontario.
answer_key: C
explanation: C 以两个专有名词做关键词定位至最后一句，说 E 的 residence time 是最小的，尽管 E 面积比 O 大，但 volume 小，所以正确答案是 C，注意不要深挖一层选 B，因为 shallow 是 volume 少的原因，但它不能直接导致 residence time 变小。但说实话，个人最初选的是这个答案，后来仔细琢磨下应该是 C 更对点儿吧，比较直接。A 明显不是作者强调的点，D 没说
*B 是错误选项是因为文章说的是 E 这个湖浅，与文章内容相悖，所以错误。

question_stem: Why does the author discuss the Great Lakes in paragraph 5?
choices: tag: A
text: To demonstrate the extent to which residence times vary from lake to lake

tag: B
text: To illustrate how residence times are calculated for specific lakes

tag: C
text: To argue that the residence time of a lake increases with area

tag: D
text: To emphasize that Lake Tahoe' s residence time is unusually long
answer_key: A
explanation: A 修辞目的题，先看例子所在句，发现整句话都是例子，所以往前看，很悲剧，前一句也是例子，所以看开头句，开头句说 residence time 变化很大，也就是说，后面举了一大堆湖都是为了说它们的 residence time 不同，从而证明变化很大，所以正确答案是 A，注意提到五大湖不是为了跟 T 湖对比，因为 T 湖本身也是个例子，所以 D 错，其他的更不靠谱.

question_stem: The word "further" in the passage is closest in meaning to
choices: tag: A
text: expected

tag: B
text: additional

tag: C
text: serious

tag: D
text: unfortunate
answer_key: B
explanation: B further: 进一步的，更多的。A 是预期的，B 是额外的，附加的，C 是严肃的，D 是不幸的。所以正确答案是 additional。

question_stem: According to paragraph 6, which of the following explains the increase in residence time of some lakes of northwestern Ontario?
choices: tag: A
text: The amount of water flowing into the lakes has increased.

tag: B
text: The rate of evaporation has decreased more sharply than the amount of rainfall.

tag: C
text: The renewal of the lakes' water has slowed due to changes in climate.

tag: D
text: Plants have required less water from the lakes
answer_key: C
explanation: C 以 northwestern Ontario 和 increase 双关键词定位至倒数第二句，但这句话只是说增加，没给出具体的解释，往下看。下一句说 slowing down of water renewal 会带来一系列后果，也就是前文说的降水减少也好蒸发量增加也罢都属于 renewal 减慢，所以正确答案是 C，其他答案都没说.

question_stem: According to paragraph 6, residence time is affected by all of the following EXCEPT
choices: tag: A
text: amount of rainfall

tag: B
text: rate of evaporation

tag: C
text: temperature of surrounding air

tag: D
text: concentration of chemicals in lake water
answer_key: D
explanation: D EXCEPT 题，排除法。A 的 rainfall，B 的 evaporation 和 C 的 temperature 做关键词都可以定位至第四句，都正确，不选；而 D 的 chemicals 做关键词定位至最后一句，但 chemical 的浓度是 renewal 减慢带来的影响，不是 residence time 的决定因素，错，选。而且根据上题也可以直接得出这道题的答案是 D.