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

A.there is evidence of diminishing marginal utility.

B.it is optimal for the consumer to have 3 cups of coffee.

C.the price of a cup of coffee must be relatively low.

D.it is optimal for the consumer to have 4 cups of coffee.

正确答案：there is evidence of diminishing marginal utility.