Suppose that an office has one secretary who can put up to two callers on hold while speaking to a third caller. (If two callers are on hold, additional callers will get a busy signal and will not call back.) If the arrival rate of calls follows a Poisson distribution with a mean rate of 20 per hour and the average length of a telephone conversation is 2 minutes, the average number of callers who will be on hold is approximately:

a. 2.0000

b. 1.3333

c. 1.0154

d. 0.4308

e. None of the above