Suppose That a Firm's Total Costs of Production Are 0

Multiple Choice

Suppose that a firm's total costs of production are 0 at an output of zero, 10 at an output of 1, 20 at an output of 2 units, 30 at an output of three units, 35 at an output of four units and 37 at an output of five units. At which number of units are marginal and average costs equal?

A) The first unit.
B) The fifth unit.
C) The third unit.
D) At the first, second and third units.