Show layer when two items are dropped

See file attached. This interaction requires the user to drag and drop two items to reveal a layer. There are 16 possible combinations (layers). They all work except 'c3' dominates when the number 3 is placed on it's drop target. Go through them in sequence and you will see. But I can't decide if either (a) I have a mistake in the triggers - if so I CANNOT find it or (b) it's a glitch? If so, can a SL staff person help please?

It does correct itself if you reset the letter item...but this is not a good solution in this particular learning context.

Any ideas?

Thanks for checking it out. Bonus is you get a cool interaction you can keep!