At a telephone enquiry system, the number of phone calls regarding relevant enquiry follow Poisson's distribution with an average of 5 phone calls during 10 min time interval. The probability that there is atmost one phone call during a 10 min time period, is [AIEEE 2006, 4, 5M] 5 6 6 (d) 5 55 6 plin (a). 5 (b)- 6 (c)