People supply their resources to the highest-paying alternative, even if working conditions are not equally attractive

There are four lighthouses on the island of Sand. If this is the efficient quantity of lighthouses, ________

A) the marginal social benefit of the fourth lighthouse equals its marginal social cost B) total social benefit is zero C) marginal social benefit minus marginal social cost is a maximum D) Both answers A and C are correct.

That is capital deepening? How does it affect labor productivity?