the length of time a given molecule of water remains in a lake？
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.

坑爹的or 逻辑。。。

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。