DISCRETE MATHEMATICS Note: There are 19 questions in total, cach 4 points for the first 13 questions and each 8 points for the remaining. For questions 1 through 13, each question has one answer. For questions 14 through 19, each question can have one answer, two answers, or more than two answers. One quarter of points will be deducted for wrong answers. 【題組】
19. Let P, Q, and R be propositions. Which of the following are tautologies? (A) P V Q V R;
(B)(P→Q)≡(-Q→-P); (C) (D)