Let p(n) be the statement that a postage of n stamps can be formed using just 4-cent and 7-cent stamps. the parts in this exercise outline an induction proof that p(n) is true for n ≥ 18. (a) show the statement p(18) is true, completing the basis step of the proof. (b) what is the inductive hypothesis of the proof? (c) what do you need to prove in the inductive step. (d) complete the inductive step for k ≥ 18. hint: you may discuss two cases, when there is at least one 7-cent stamp for p(k), and when k is a multiple of 4.