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). What is the minimum number of clerks needed to keep the average time in the system under 5 minutes?

a. 2
b. 3
c. 4
d. 5
e. 6