Lennie is a worker for Clean&Clean Co., a car wash company. It seems that the company’s business is pretty good. We observed that on average 15 customers arrive every hour. The arrival is Poisson distributed. The average car washing time is 2 minutes per car, following an exponential distribution. What is the average number of cars waiting to be washed? Select one: a. 0 b. 1/2 c. 4/3 d. 2 e. 3