An instance of a relational scheme R(A, B, C) has distinct values for attribute A. Can you conclude that A is a candidate key for R?

Let $$p$$ and $$q$$ be propositions. Using only the truth table decide whether $$p \Leftrightarrow q$$ does not imply $$p \to \sim q$$ is true or false.

Let A and B be any two arbitrary events, then, which one of the following is true?

Some group (G, o) is known to be abelian. Then, which one of the following is true for G?