A one-hour film processing store has a single machine that processes film as it arrives. The mean arrival rate of film is 60 rolls per hour and follows a Poisson Distribution. The mean service rate (development rate) of the film is 80 rolls per hour and follows an Exponential Distribution. What is the mean number of rolls of film that are in the store, waiting to be processed? (This does not include any film that is currently being processed on the machine.)

A.20

B..75

C.4.25

D.6.125

E.none of the above