removeMovieClip() plz help

i am making a a game where circles are coming and u dodge them . anyway i create circles and it increments the name with x++;

so after i create it i do the onEnterFrame function to make my circle move. Well after it goes of the screen i want it to be removed cause its starts lagging after a while. But wen i try this code it only removes the first circle but doesnt affect the other circles. can someone plz help me.

_root[“enemy_circle” + x].onEnterFrame = function(){
this._y += _y + _root.speed;
if(this._y > 200){
so basicly i have like 10 circles coming down it will only remove the first one and the other ones wont be removed.

Can someone tell me the problem why its not removing the other circles?