-Refer to the Above Figure $S _ { 1 }$ Is the Supply Curve That Includes Only Private Costs

-Refer to the above figure. $S _ { 1 }$ is the supply curve that includes only private costs. $S _ { 2 }$
Is the supply curve that includes social costs.If the firm sets output by focusing on private costs,the per-unit external cost will equal

A) $P _ { 2 }$ .
B) $\mathrm { P } _ { 4 }$ -
$P _ { 2 }$ .
C) $\mathrm { P } _ { 4 }$ -
$P _ { 1 }$ .
D) $P _ { 2 }$ -
$P _ { 1 }$ .