set theory: e and f only. please use the format to answer and explain your proof.

theorem:

proof:

6. * Suppose a, b, c, and d are positive integers. Prove that (a) 2a - 1 is odd. (b) if a is even, then a + 1 is odd. (c) if a is odd, then a + 1 is even. (d) a(a + 1) is even. (e) 1 divides a and a divides a. (f) if a divides b, then a sb. (g) if a divides b, then a divides bc. * *