Refer to the Following:
Following Is a Firm's Expansion Path

Multiple Choice

Refer to the following:
Following is a firm's expansion path. The price of capital is $5 per unit; the price of labor is $2 per unit.

$\begin{array}{ccc}&\text { Optimal Imput Choice }\\\text { Units of Output } & \text { Units of Capital }&\text { Units of Labor } \\10 & 6 &5 \\20 & 8 & 10 \\30 & 13 & 20 \\\end{array}$
-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