First, put bound values on the dropdown items in the "object" pallet "Binding" tab:
Like:
Yes has bound value = 1
No has a bound value = 0.
Then,
Put on the "exit" event on both dropdown boxes and choose "JavaScript" for the scripting language:
//////////////////////////////
if (this.rawValue == "1" && B.rawValue == "1")
{
C.presence = "visible";
}
else
{
C.presence = "invisible";
}
//////////////////////////////
modify the first line to refenence A.rawValue when you add this script to the B dropdown box.
That should do it.
Good luck,
Stephen