how do I tell wether the given se of numbers forms a group with respect to the given operation?

a) {-1,1}, multiplication

b) {-1,0,1}, addition

c) Z, subtraction

d) {2^m : m is a member of Z}, multiplication

e) the set of matrices ,... a,b is a member of R, (a^2) + (b^2) >0, multiplication

a b

-b a

I think you have to tell if they are associative or not, and I think (a) is, but how do you know for sure???