A computer services center has a problem with "malicious" cotnputer usage. The center's director has decided to hire additional personnel to monitor computer usage. In order to minimize the total cost of the malicious usage, the director should hire the number of monitors at which

Select one:

a$.$ the marginal revenue from the last monitor equals the marginal cost of the last monitor.

b$.$ the decrease in the cost of malicious usage from the last monitor hired equals the cost of hiring the last monitor.

c$.$ the marginal benefit per dollar is equal across monitors.

d$.$ both $b$ and $c$

e$.$ All of the above.