A binary operation * on a set S is said to be associative if and only if for any element a, b, ceS Select one: a. a∗b∗c=c∗b∗a b. a∗b=b∗a c. (a∗b)∗c=a∗(b∗c) d. none of these If S is a set having identity element " e " with respect to the binary operation * and corresponding to each element " a " in S, there exist an element " b" in S such that Select one: a. a∗b=b∗a=e b. a+b=b+a=e c. a∗e=b∗e=e d. a/b=b/a=e