When there is an external benefit, the unregulated market

The Millennium Development Goals indicate that

a. extreme poverty can only be eliminated by introducing democratic norms. b. the alleviation of extreme poverty is a moral responsibility that cannot be ignored. c. foreign direct investment actually ends up increasing global poverty. d. there is little evidence that institutionalized programs reduce global poverty.

In the above figure, in order to achieve efficiency, a tax of ________ per unit is needed

A) $3 B) $6 C) $0 D) $2