[FlashCS4ActionScript2.0] Need help with conditional statement to trigger an event

I’m a newbie to actionscript, so please bear with my (semi-)stupid question:
I want to trigger an event when 4 buttons are being set to visible(originally set to ._visible=false at the beginning of the movie)
say, the instance names of the buttons are “a”,“b”,“c”,and “d”, and their visibility is triggered by other events.When all of them are triggered/being set to visible, I want a movieclip to play (say, the name of the movieclip is “animation”)
I tried using the if statement and came up with the following code:

if(a._visible==true && b._visible==true && c._visible==true && d._visible==true){
animation.gotoAndPlay(1);
}

I tried using that code, but nothing happened to the movie…
all the events and buttons are located in the main timeline…
HELLLPP? this is for a school assignment that is due ASAP…:puzzle::puzzle: