How to Select a Choice in a dropdown upon Another dropdown input.

How to Select a Choice in a dropdown upon Another dropdown input.

Hi i have two dropdown's 

dropdown1 represents Product and dropdown2 represents Units.

If i select choice Description as "Wall Concrete Pouring" then i want Units to be "Cubic meters" .

If (input.Description = "Wall Concrete Pouring")
{
Units = "Cubic Meters";
}

It does not work.

Any help!