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.

定位句中带了or：

it might...or it might...

前后是两种不同情况。

看选项：

Question 1 of 5

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

只有C选项是说了两种情况。排除法直接排除其他。