複選題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)

複選題3. Suppose we have two functions fand g, withf(1)=3,f(2)=1,f(3)=4,f(4)=6, g(1)=5,g(2)=3,g(3) =4,g(4)=1,g(5) =3,and g(6) = 2. Which is the correct mapping of the composite function g o f? (A) go f(1) = 5； (B) go f(2) = 3； (C) go f(3) = 1； (D) g o f(4) = 6.