Mall owner: Our mall's occupancy rate is so low that we are barely making a profit. We cannot raise rents because of unacceptably high risk of losing our tenants. On the other hand, a mall that is fully occupied costs as much to run as one with a rental space here and a rental space there free. Clearly, therefore, to increase profits we must sign up new tenants.

The occupancy rate is low so profits are low.
Rents cannot be raised because tenants will leave.
Cost does not increase if more tenants come. (cost of running a fully occupied mall is the same as the cost of running a mall with some vacant spaces)

So we should sign up new tenants.

The sentence is considering the cost of the proposal too. If more tenants are signed, cost of running the mall will not increase so we should sign up more tenants.

Answer: A