Answer the Question on the Basis of the Following Cost

Answer the question on the basis of the following cost data for a purely competitive seller: $\begin{array}{ccc}\begin{array}{c}&\text { Total } \\\text { Total }&\text { Fixed } \\\text { Output }&\text { Cost }\\\hline0&\$50\\1 & 50 \\2 & 50 \\3 & 50 \\4 & 50 \\5 & 50 \\6 & 50\end{array}\begin{array}{cc}\text { Total } &\\\text { Variable }&\text { Total } \\\text { Cost }&\text { Cost }\\\hline\$ 0 & \$ 50 \\70 & 120 \\120 & 170 \\150 & 200 \\220 & 270 \\300 & 350 \\390 & 440\end{array}\end{array}$ Refer to the data.The marginal cost of the fifth unit of output is:

A) $80.
B) $90.
C) $50.
D) $20.