Following Is a Firm's Expansion Path $\quad$ $\quad$ $\quad$

Multiple Choice

Following is a firm's expansion path.The price of capital is $5 per unit; the price of labor is $2 per unit. $\quad$ $\quad$ $\quad$ $\quad$ $\quad$ $\quad$ $\quad$ $\quad$$\text { Ontimal Input Choice }$
$\begin{array}{c}\begin{array}{c}\text {Units of Output}\\10\\20\\30\end{array}\begin{array}{c}\text {Units of Capital}\\6\\8\\13\end{array}\begin{array}{c}\text {Units of Labor}\\5\\10\\20\end{array}\end{array}$
How much does the 10th unit of output add to long-run total cost?

A) zero
B) $3
C) $4
D) $40
E) none of the above