Which of the following functions are the isomorphisms? a) :(Z,+)(Z,+) defined by o(n) = 2n, for xeZ b) : (Q,+)→(Q,+) defined by (x)=, forx Q c) : (2,+)-(2+) defined by o(n) n+1, for xeZ d) :(Q)(Q) defined by (x) = x², for xeQ Answer CA B