A waiting-line system that meets the assumptions of m/m/s (multi-channel) has λ = 20 per hour, μ = 4 minutes, and m = 2. For these values, po is approximately 0. 23077, and ls is approximately 2. 05128 and Ls is approximately 2. 5128. What is the average time a unit spends in the queue (in minutes)?

a. 3.2 minutes
b. 053 minutes
c. 12 minutes
d. 7.2 minutes
e. 2.4 minutes