Determine which of the following are propositions and which are not. For those that are, determine their truth values.

(a) x + y = y + x for all x, y ∈ R.

(b) AB = BA, where A and B are square matrices.

(c) Academics are absent-minded.

(d) I think that the world is flat.

(e) Go fetch a policeman.

(f) Every even integer greater than 4 is the sum of two prime numbers. (This is Goldbach’s conjecture.)