show that the statements p(18), p(19), p(20), and p(21) are true, completing the basis step of the proof. (please enter your answers as numeric values only.) (you must provide an answer before moving to the next part.) p(18) is true, because 18 cents of postage can be formed from 1 4-cent stamps and 1 7-cent stamps. p(19) is true, because 19 cents of postage can be formed from 1 4-cent stamps and 0 7-cent stamps. p(20) is true, because 20 cents of postage can be formed from 5 4-cent stamps and 0 7-cent stamps. p(21) is true, because 21 cents of postage can be formed from 0 4-cent stamps and 3 7-cent stamps.