Customers enter the waiting line at a cafeteria's only cash register on a first-come, first-served basis. The arrival rate follows a Poisson dis- tribution, while service times follow an exponential distribution. If the average number of arrivals is 6 per minute and the average ser- vice rate of a single server is 10 per minute, what is the average num- ber of customers in the system?

a) 0.6

c) 1.5

e) 1.0

b) 0.9

d) 0.25