Customers enter the waiting line to pay for food as they leave 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, on average, how much

time will elapse from the time a customer enters the line until he $/$ she leaves the cafeteria?

A $)0\mathrm{.}5021$ ?minute

B $)0\mathrm{.}6667$ ?minute

C $)0\mathrm{.}3333$ ?minute

D $)0\mathrm{.}7565$ ?minute