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

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.

$\begin{array}{ccc}&\text { Optimal Imput Choice }\\\text { Units of Output } & \text { Units of Capital }&\text { Units of Labor } \\10 & 6 &5 \\20 & 8 & 10 \\30 & 13 & 20 \\\end{array}$
-When output is 20 units, what is long-run average cost?

A) $2
B) $3.50
C) $4
D) $4.50
E) none of the above