List all the elements in each of the following subgroups a.) The subgroup of Z24 generated by 15. b.) All the subgroups of Z12. c.) The subgroup generated by 5 in (Z18). d.) The subgroup of CX generated by 2i. 18. Let p and q be two distinct primes and let r>0 € Z a.) How many generators does Zpq have? b.) How many generators does Zpr have? c.) Prove that Zp has no nontrivial subgroups.