During a Professor's Office Hours,students Arrive,on Average,every Ten Minutes

Multiple Choice

During a professor's office hours,students arrive,on average,every ten minutes.Assume that the distribution of the time between arrivals follows an exponential distribution.Suppose that a student has just left. What is the probability that the professor has more than 20 minutes before the next student shows up?

A) 0.8187
B) 0.8647
C) 0.1353
D) 0.1813