Suppose a and b are generators of a group G, that is, G = (:a:) = (:b:). Show that the function phi: G -> G given by phi(a^(k)) = b^(k) is an isomorphism. Which of the following options correctly describes the function phi?

A. phi(a^(k)) = b^(k) for all k in Z
B. phi(a^(k)) = b^(k) for all k in N
C. phi(a^(k)) = b^(k) for all k in Q
D. phi(a^(k)) = b^(k) for all k in R