16. (2001 World Inter-City Competition) B has to guess A's two-digit number. If both digits are correct, or one is correct and the other differs by 1, A says “near”; otherwise, A says “far”. For example, if A's number is 65, and B guesses 65, 64, 66, 55, or 75, A says “near”; otherwise, A says “far”.
(a) Prove that B cannot guarantee to know A's number in just 18 guesses;
(b) Find a method for B to guarantee knowing A's number in 24 guesses;
(c) Is there a method for B to guarantee knowing A's number in 22 guesses?