How to disable pressing multiple keys at once?

[vince677]

I’m trying to give user an option to choose one from 4 buttons, but when multiple keys are pressed at once, it executes more functions than one.

keyQ.onDown.add(upA, this);
keyW.onDown.add(upB, this);
keyE.onDown.add(upC, this);
keyR.onDown.add(upD, this);
keyT.onDown.add(dismiss, this); 

I tried to use isDown like this, but It isn't working outside update function

if (this.keyQ.isDown)
{
	upA();
}
else if (this.keyW.isDown)
{
	upB();
}