Consider the Following Production and Cost Schedule for a Firm

Consider the following production and cost schedule for a firm.The first column shows the number of units of a variable factor of production employed by the firm. $$\begin{array} { | c | c | c | } \hline \begin{array} { c } \text { Total Number of } \\\text { Units of the } \\\text { Factor }\end{array} & \begin{array} { c } \text { Total Number of } \\\text { Units of Output }\end{array} & \begin{array} { c } \text { Price per Unit } \\\text { of Output }\end{array} \\\hline 10 & 20 & \$ 10 \\\hline 11 & 44 & \$ 10 \\\hline 12 & 64 & \$ 10 \\\hline 13 & 78 & \$ 10 \\\hline 14 & 84 & \$ 10 \\\hline 15 & 86 & \$ 10 \\\hline\end{array}$$ TABLE 13- 1
-Refer to Table 13- 1.The marginal revenue product of the 15th unit of the factor is

A) $60.
B) $820.
C) $150.
D) $20.
E) - $20.