Why do larger gas molecules move slower at the same temperature?

[Linzy]

Homework Statement

Four Containers, kept at the same temperature, have different molecules (A,B,C,D) in gaseous form. The size of the molecules indicates their mass (larger the size, greater the mass). Recalling that Kinetic Energy (KE) is given by KE=.5mv², choose the container with the smallest average velocity molecules
http://img171.imageshack.us/img171/7545/problem3ul3.jpg

Homework Equations

The Attempt at a Solution

I believe it is B, because larger molecules move slower, but I can't seem to find a way to prove it on my homework.

 

[Kurdt]

Does it ask you to prove it?