During the early morning hours, customers arrive at a branch post office at an average rate of 63 per hour (Poisson), while clerks can provide services at a rate of 21 per hour. If clerk cost is $26.26 per hour and customer waiting time represents a cost of $26 per hour, how many clerks can be justified on a cost basis a. 8 b. 5 c. 4 d. 7 e. 6