Answer the Question on the Basis of the Accompanying Table

Answer the question on the basis of the accompanying table that shows average total costs (ATC) for a manufacturing firm whose total fixed costs are $10. $\begin{array}{|c|c|}\hline \text { Output } & \text { ATC } \\\hline 1 & \$ 40 \\\hline 2 & 27 \\\hline 3 & 29 \\\hline 4 & 31 \\\hline 5 & 38 \\\hline\end{array}$ The marginal cost of the fourth unit of output is

A) $2.
B) $12.
C) $37.
D) $16.