The Table Below Shows a Competitive Firm's Short-Run Production Function $\begin{array}{cccc} \text {Units of Labor } & \text { Units of Output} \\3 &370\\4 &490\\5 &570\\6 &600\\7&620\\\end{array}$

Multiple Choice

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. $\begin{array}{cccc} \text {Units of Labor } & \text { Units of Output} \\3 &370\\4 &490\\5 &570\\6 &600\\7&620\\\end{array}$

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