Vinnie Is Looking for an Apartment in Regina

Vinnie is looking for an apartment in Regina.In Regina,75% of the two-bedroom,one-bath apartments cost $800 a month and 25% cost $600.Vinnie would prefer to find an apartment that rents for $600.The marginal cost of looking at the first apartment is $15,the marginal cost of looking at the second apartment is $30,and so on.
-Refer to the information above.The marginal cost of looking at the fourth apartment is

A) $0.
B) $15.
C) $45.
D) $50.
E) $60.