7. During the early morning hours, customers arrive at a branch post office at an average rate of 75 per hour (Poisson), while clerks can provide services at a rate of 25 per hour. If clerk cost is $38.54 per hour and customer waiting time represents a cost of $41 per hour, how many clerks can be justified on a cost basis?

a. 5

b. 4

c. 6

d.7

e.8