Xavier Notices That the Marginal Utility of Working with a Tutor

Xavier notices that the marginal utility of working with a tutor seems to fall with each hour the tutor helps him study. If Xavier keeps the tutor until his grade actually begins to fall, his marginal utility for the last hour of tutoring will be:

A) negative.
B) positive, but rising more slowly.
C) zero.
D) immeasurable.