Here are some expressions

1)ðr^2l
2)2ðr^2
3)4ðr^3
4)abrl
5)abl over r
6)3(a^2+b^2)r
7)ðrl

The letters r, l, a and b represent lengths. ð, 2, 3 and 4 are numbers that have no dimensions. Three of the expressions represent volumes. Which ones are they?

please could you help
thanks in advance :)