The average time between two arriavals of customers at a counter in a ready - made garment store is 4 min. The average time of the counter clerk to serve the customer is 3min. The arrivals are distributed as per Poisson distribution and the services are as per the exponential distribution. The probability that a customer arriving at the counter will have to wait is
(a) Zero (b) 0.25
(c) 0.50 (d) 0.70