Sis the set of prime numbers that are less than 15 a. {2, 3, 5, 7, 11, 13} b.{2, 3, 5, 7, 9, 11, 13} c.{1, 3, 5, 7, 9, 11, 13} d.{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}