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. $$\begin{array} { | c | c | c | } \hline \begin{array} { c } \text { Total Numb er of } \\\text { Units of theF actor }\end{array} & \begin{array} { c } \text { Total Number of } \\\text { Units of Output }\end{array} & \begin{array} { c } \text { Price per Unit } \\\text { of Output }\end{array} \\\hline 10 & 20 & \$ 10 \\\hline 11 & 44 & \$ 10 \\\hline 12 & 64 & \$ 10 \\\hline 13 & 78 & \$ 10 \\\hline 14 & 84 & \$ 10 \\\hline 15 & 86 & \$ 10 \\\hline\end{array}$$ TABLE 13- 1
-Refer to Table 13- 1.The marginal revenue product of the 14th unit of the factor is

A) $700.
B) - $60.
C) $140.
D) $60.
E) $840.