During the morning rush hour, the time between calls for assistance to a towing service averages 15 minutes and follows an exponential distribution. Over the morning rush hour:
a) More than 50% of calls will be less than 15 minutes apart.
b) Exactly 50% of calls will be less than 15 minutes apart.
c) Less than 50% of calls will be less than 15 minutes apart.
d) The distribution of call times cannot be determined without additional information.