darg and drop free form snapping prevention


I created a drag and drop question using free forms. I wanted the students to drag and drop one answer to each gap and never be able to change it so I changed the states of all the answers to Disabled once they are dropped in any of the gaps. But, when I tried the while exercise, I discovered that my students would be able to change their answers! Once they drag and drop any other answer to the same gap they have already filled in with an answer, the previous answer snaps back to the container and its state stays disabled! So the answers could be changed! And to make it worse, once these answers snap back to the container, my students can't use them! How could I fix this? Anyone knows?

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