The set-builder form for the following set (1, 5, 15, 25,.) is : Select one: a. (x: either x=1 or x=5n, where n is a integer) b. (x: x=5n, where n is a natural number) C. (x: either x-1 or x=5n, where n is an odd natural number) d. (x: either x=1 or x=5n, where n is a real number)