The Table Below Shows the Total Production for Varying Numbers

The table below shows the total production for varying numbers of hours worked producing yo- yos. The firm sells its product and hires its workers in competitive markets. $$\begin{array}{l}\begin{array} { | l | l | l | l | } \hline \begin{array} { l } \text { Number of } \\\text { Hours Worked }\end{array} & \begin{array} { l } \text { Number of } \\\text { Yo- Yos } \\\text { Produced }\end{array} & \begin{array} { l } \text { Wage Rate } \\\text { per Hour } ( \$ ) \end{array} & \begin{array} { l } \text { Price of Yo- Yos } \\( \$ ) \end{array} \\\hline 10 & 60 & 10 & 5 \\\hline 11 & 70 & 10 & 5 \\\hline 12 & 78 & 10 & 5 \\\hline 13 & 84 & 10 & 5 \\\hline 14 & 88 & 10 & 5 \\\hline 15 & 90 & 10 & 5 \\\hline\end{array}\\\text { TABLE 13- } 4\end{array}$$
-Refer to Table 13- 4. What is the marginal cost of the variable factor employed by this firm?

A) $0
B) $5
C) $20
D) $30
E) $10