The Grand Movie Theater has one box office clerk. On average, each customer that comes to see a movie can be sold its ticket at the rate of 6 per minute. For the theater's normal offerings of older movies, customers arrive at the rate of 3 per minute. Assume arrivals follow the Poisson distribution and service times follow the exponential distribution. What is the average time a customer spends in the waiting line?

a. 0.2051

b. 0.1667

c. 0.3333

d. 0.0937