Please show me how you get the answer:

Exerting the appropriate effort to make a sale versus shirking will cost a salesman the equivalent of $40 and will increase her sales effectiveness from 20% to 40%:

a. You should offer a commission only if the contribution margin is greater than $200.

b. You should pay your sales force an hourly wage only if the contribution margin is greater than $200.

c. You should offer a commission only if the contribution margin is greater than $100.

d. You should pay your sales force an hourly wage only if the contribution margin is greater than $100.