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

Multiple Choice

DeShawn's Detailing is a service that details cars at 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