Drag and Drop: Normal, Correct, and Incorrect State

Hi Folks,

I'm very, very new here and really appreciate any advice you can offer!

What I'm trying to do is design a slide, where a user chooses a slang term (from a list of items) and matches it to the appropriate formal language term (for example, matching the slang "hot" to the formal "beautiful"). 

What I'd like to do is display a change state for the "drop box" to show correct or incorrect, based on the users input. This is easy enough, but what I can't seem  to figure out is:

How to change the state of the drop box back to normal, if the user drags the correct or incorrect item OFF the box
Allow only one item  at at time, to be added to each drop box