Suppose That for a Particular Firm the Only Variable Input

Suppose that for a particular firm the only variable input into the production process is labor and that output equals zero when no workers are hired.In addition,suppose that marginal cost when three workers are hired is $40 and the average total cost when three workers are hired is $50.What is the total cost of production when three workers are hired?

A) $50
B) $90
C) $120
D) $150