Suppose That the Marginal Benefit of Having a Security Guard

Suppose that the marginal benefit of having a security guard at the airport is MB₁,at the bank is MB₂,and at the mall is MB₃.The marginal cost of assigning security guard is the same for all three places,and it is represented by MC₂.
Suppose all three locations hire the efficient number of guards,and that the marginal cost of having a security guard equals his or her wage.Efficiency would also require that

A) all security guards be paid the same wage regardless of where they work.
B) more security guards be hired at the location at which marginal cost is lowest.
C) more security guards would be hired at the location at which marginal cost is greatest.
D) security guards at the airport be subsidized because private markets will under-provide them.