-If This Firm Were a Perfect Competitor Selling Its Entire

Multiple Choice

$$\begin{array}{l}\begin{array} {ccc } \text { Units of Input}&\text { Units of Output }\\1 & 4 \\2 & 9 \\3 & 13 \\4 & 15 \\5 & 16 \\6 & 16 \\7 & 15\end{array}\end{array}$$
-If this firm were a perfect competitor selling its entire output at a price of $10,the marginal revenue product of the fourth unit of output would be

A) $0.
B) $10.
C) $15.
D) $20.
E) $150.