Refer to the Following:
the Table Below Shows a Competitive $\begin{array}{ll} \text {Units of Labor }& \text {Units of Output }\\3 & 370 \\4 & 490 \\5 & 570 \\6 & 600 \\7 & 620\end{array}$

Refer to the following:
The table below shows a competitive firm's short-run production function. Labor is the firm's only variable input, and market price for the firm's product is $2 per unit.
$\begin{array}{ll} \text {Units of Labor }& \text {Units of Output }\\3 & 370 \\4 & 490 \\5 & 570 \\6 & 600 \\7 & 620\end{array}$
-How much does the fifth unit of labor add to the firm's total revenue?

A) $160
B) $80
C) $60
D) $40
E) $10