Which of the sets in Exercises 1 through 6 are subgroups of the group C^* of nonzero complex numbers under multiplication?

Data from Exercises 1-6

1. R

2. Q⁺

3. 7Z
4. The set iR of pure imaginary numbers including 0 

5. The set πQ of rational multiples of π 

6. The set {πⁿ | n ∈ Z}