During the Early Morning Hours, Customers Arrive at a Branch

Multiple Choice

During the early morning hours, customers arrive at a branch post office at the average rate of 45 per hour (exponential interarrival times) , while clerks can handle transactions on an average of 4 minutes each (exponential) .
If clerk cost is $30 per hour and customer waiting time represents a cost of $20 per hour, how many clerks can be justified on a cost basis?

A) 2.
B) 3.
C) 4.
D) 5.
E) 6.