You can only make the entire List visible, invisible ....you cannot make individual list items visible, invisible. You can automatically select the item from the list you want beased on sopmething else in the form though.
If I interpret your answer correctly, then what I'd like to do should be possible:
Only if a certain item on "DropListA" is selected, "DropListB" should be made visible. So an item in DropListA should Trigger the event to make DropListB visible. With what code could I accomplish such thing?