If the Marginal Utility of the 3^rd Cup of Coffee

If the marginal utility of the 3^rd cup of coffee is 23 and the marginal utility of the 4^th cup is 15, then:

A) it is optimal for the consumer to have 3 cups of coffee.
B) the price of a cup of coffee must be relatively low.
C) it is optimal for the consumer to have 4 cups of coffee.
D) there is evidence of diminishing marginal utility.