Setting movieclips from library

1

i have three movieclips in my library(Ground1, Ground2, Ground3).

depending on the game level i want to use a different one but i reference the ground movieclip through out the game. how can i declair the variable(ie
var ground:Ground1;) so i can set ground to a different library clip like this:

if(level == 1){
ground = new Ground1();
}
if(level == 2){
ground = new Ground2();
}
if(level == 3){
ground = new Ground1();
}
ground.scaleX = 10;
ground.scaleY = 10;
worldSprite.addChild(ground);