If total utility is increasing, then marginal utility

A) must be declining.
B) must be increasing.
C) must be increasing at an increasing rate.
D) may either be increasing or decreasing, but it must be greater than zero.

When total utility is increasing, it means that the addition of more units of a good or service is still providing positive utility, hence the marginal utility must be greater than zero. However, it does not specify whether the rate of increase in marginal utility is increasing, decreasing, or constant, only that it is positive.