Suppose the Marginal Cost of the 1^st Hour of Talking

Suppose the marginal cost of the 1^st hour of talking on the phone is $50, the marginal cost of the 2^nd hour is $75, and the marginal cost of the 3^rd hour is $105. In this case, the total cost of talking on the phone for 3 hours is:

A) $315
B) $105
C) $230
D) $150