If Jane works for 6 hours she can rent out 9 apartments, and if she works for 7 hours she can rent out 12 apartments. The marginal benefit of Jane’s 7th hour of work equals:

If Jane works for 6 hours she can rent out 9 apartments, and if she works for 7 hours she can rent out 12 apartments. The marginal benefit of Jane’s 7th hour of work equals:
A.3 apartments
B.9 apartments
C.12 apartments
D.1 apartment
正确答案：3 apartments