Use the Following to Answer Questions: Table: Costs of Antibiotics

Use the following to answer questions: Table: Costs of Antibiotics $$\begin{array} { c c c c c } \hline \begin{array} { c } \text { Quantity of } \\\text { Antibiotics }\end{array} & \begin{array} { c } \text { Marginal Benefit } \\\text { to Buyers }\end{array} & \begin{array} { c } \text { Marginal Cost } \\\text { to Sellers }\end{array} & \begin{array} { c } \text { External } \\\text { Cost }\end{array} & \begin{array} { c } \text { Marginal Social } \\\text { Cost }\end{array} \\\hline 1 & \$ 25 & \$ 5 & \$ 10 & ? \\2 & \$ 20 & \$ 10 & \$ 10 & ? \\3 & \$ 15 & \$ 15 & \$ 10 & ? \\4 & \$ 10 & \$ 20 & \$ 10 & ? \\5 & \$ 5 & \$ 25 & \$ 10 & ? \\\hline\end{array}$$
-(Table: Costs of Antibiotics) Refer to the table. The marginal social cost of the fifth unit is:

A) $15.
B) $5.
C) $35.
D) $30.