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