Find if possible a choice function for each of the following collections, without using the choice axiom:
a. The collection A of nonempty subsets of Z+ (positive integers)
b. The collection B of nonempty subsets of Z (integers)
c. The collection C of nonempty subsets of the rational numbers Q
d. The collection D of nonempty subsets of X^(omega), where X = {0, 1}