Platform problem

I am using actionscript 2.0 on Flash CS3.

The code looks like this:

onClipEvent (load) {
var MovingFloor:MovieClip = _root.MovingFloor;
var NormalFloor:MovieClip = _root.NormalFloor;
var grav:Number = 0;
var gravity:Number = 2;
var speed:Number = 3.5;
var maxJump:Number = -12;
var touchingGround:Boolean = false;

}
onClipEvent (enterFrame) {
_y += grav;
grav += gravity;

while (MovingFloor.hitTest(_x, _y, true)) {
_y -= gravity;
grav = 0;
}
if (MovingFloor.hitTest(_x, _y+5, true)) {
touchingGround = true;
} else {
touchingGround = false;
}
if (Key.isDown(Key.RIGHT)) {
_x += speed;
}
if (Key.isDown(Key.LEFT)) {
_x -= speed;
}
if (Key.isDown(Key.UP) && touchingGround) {
grav = maxJump;
}
if (MovingFloor.hitTest(_x+(_width/2), _y-(_height/2), true)) {
_x -= speed;
}
if (MovingFloor.hitTest(_x-(_width/2), _y-(_height/2), true)) {
_x += speed;
}
if (MovingFloor.hitTest(_x, _y-(height), true)) {
grav = 3;
}


while (NormalFloor.hitTest(_x, _y, true)) {
_y -= gravity;
grav = 0;
}
if (NormalFloor.hitTest(_x, _y+5, true)) {
touchingGround = true;
} else {
touchingGround = false;
}
if (Key.isDown(Key.RIGHT)) {
_x += speed;
}
if (Key.isDown(Key.LEFT)) {
_x -= speed;
}
if (Key.isDown(Key.UP) && touchingGround) {
grav = maxJump;
}
if (NormalFloor.hitTest(_x+(_width/2), _y-(_height/2), true)) {
_x -= speed;
}
if (NormalFloor.hitTest(_x-(_width/2), _y-(_height/2), true)) {
_x += speed;
}
if (NormalFloor.hitTest(_x, _y-(height), true)) {
grav = 3;
}
}
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How can I make it so that when I am underneath the platform, I cannot
jump through it?

Thanks,
Tom.