Refer to the Following:
the Capital Stock Is Fixed at 40

Multiple Choice

Refer to the following:
The capital stock is fixed at 40 units, the price of capital is $15 per unit, and the price of labor is $80 per unit.
$$\begin{array} { c c } \text { units of } & \text { units of } \\\text { Labor } & \text { Output } \\\hline 5 & 40 \\10 & 100 \\15 & 180 \\20 & 220\end{array}$$
-How much does the 50th unit of output add to the firm's total cost?

A) $6.67
B) $10
C) $400
D) $600
E) none of the above