Let G be an Abelian group. Let H and K be subgroups of G with orders 36 and 45, respectively. Which of the following is a possible order of HK?

a) 5

b) 9

c) 20

d) 90

e) 180

This doesn't seem difficult, but I've never taken Abstract Algebra before... So I don't even know what it means to find the order of a subgroup.