A Company That Receives Most of Its Orders by Telephone

A company that receives most of its orders by telephone conducted a study to determine how long customers were willing to wait on hold before ordering a product.The length of waiting time was found to be a variable best approximated by an exponential distribution with a mean length of waiting time equal to 2.8 minutes .What proportion of callers is put on hold longer than 2.8 minutes?

A) 0.3679
B) 0.50
C) 0.60810
D) 0.6321