The marginal benefit of a glass of water depends on your circumstances. If you have just run a marathon or you have been walking in the desert sun for three hours, the marginal benefit is very high. But if you have been drinking alot of liquids recently, the marginal benefit is quite low. The point is that even the necessities of life, like water, do not always have large marginal benefits.
Work Step by Step
Read the section in the chapter that discusses about marginal benefit to get the gist of this question.