Refer to the Accompanying Table, Where Q Represents the Quantity

Refer to the accompanying table, where Q represents the quantity produced, internal cost and social cost are given for various quantities, and P represents the price consumers are willing to pay for various quantities. $\begin{array}{lll}Q&\text { Internal Cost }&\text { Social Cost }&P\\100&\$ 40 & \$ 60 & \$ 80 \\200&\$ 50 & \$ 70 & \$ 70 \\300&\$ 60 & \$ 80 & \$ 60 \\400&\$ 70 & \$ 90 & \$ 50 \\500&\$ 80 & \$ 100 & \$ 40 \\600&\$ 90 & \$ 110 & \$ 30\end{array}$ The external cost is equal to _________ per unit.

A) $60
B) $70
C) $20
D) $50
E) $30