Customers Enter the Waiting Line at a Cafeteria on a First-Come,first-Served

Customers enter the waiting line at a cafeteria on a first-come,first-served basis.The arrival rate follows a Poisson distribution,while service times follow an exponential distribution.If the average number of arrivals is four per minute and the average service rate of a single server is seven per minute,what is the average number of customers in the system?

A) 0.43
B) 1.67
C) 0.57
D) 1.33
E) None of the above