DeShawn's Detailing Is a Service That Details Cars at the Customers

DeShawn's Detailing is a service that details cars at the customers' homes or places of work. DeShawn's cost for a basic detailing package is $40, and he charges $75 for this service. For a total price of $90, DeShawn will also detail the car's engine, a service that adds an additional $20 to the total cost of the package. What is the marginal cost of adding the engine detailing to the basic detailing package?

A) $20
B) $30
C) $60
D) $60 plus the value of his time