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 waiting in line behind the person being served?

A) 0.76
B) 0.19
C) 1.33
D) 1.67
E) None of the above