If $aN= \{ ax:x\in N \}$ then $3N\cap 7N$ =

Options are 
a)3N
b)7N
c)N
d)21N

I think the answer should be 3N as 7N would contain 3N and we have to find their intersection. But the answer is 21N, and I am not able to figure out the reason for that answer.