Suppose that Save-on-Foods models their checkout process as an M/D/1 queue. The supermarket trains the employee working the checkout to serve one customer every two minutes. After collecting some data, the supermarket notices that the server is only being 50% utilized. The supermarket also sees that the average time a customer spends in the entire checkout process is 10 minutes. On average, how many customers are waiting in the queue?
A. 1
B. 2
C. 3
D. 4