By whatever means, a lake is constantly gaining 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 the average length of time that any particular molecule of water remains in the lake, and it is calculated by dividing the volume of water in the lake by the rate at which water leaves the lake. The residence time is an average; the time spent in the lake by a given molecule (if we could follow its fate) would depend on the route it took: it might flow through as part of the fastest, most direct current, or it might circle in a backwater for an indefinitely long time.

It can be inferred from paragraph 4 that the length of time a given molecule of water remains in a lake

A. depends entirely upon the average speed of a lake' s currents

B. can be measured by the volume of the lake alone

C. can be greater or lesser than the residence time

D. is similar to the length of time all other molecules remain in that lake

1)首先审题，我们知道推理对象为the length of time a given molecule of water remains in a lake;

2)然后通过定位，找到原文包含推理对象的句子，即原文最后一句话的后半句，“The residence time is an average; the time spent in the lake by a given molecule (if we could follow its fate) would depend on the route it took: it might flow through as part of the fastest, most direct current, or it might circle in a backwater for an indefinitely long time.”

3)本句话以分号分成了前后两个分句，因此前一个分句也需要分析。前一个分句，即“The residence time is an average，停留时间是平均的”，这个信息点需要记住;

4)接下来再把后半句看完。后半句，即“the time spent in the lake by a given molecule would depend on the route it took，一分子的水停留在湖里的时间取决于它所走的路线”，隐含意思即水停留在湖里的时间是不固定的，走的路线是直线，则停留时间短，走的路线是循环的，则停留时间就长;

5)结合前一个分句，可得出结论，一分子的水停留在湖里的时间可能比residence time 长，也可能比它短，故答案为C。