Restrict user to take the test again

We use transparent gifs to disable questions once they've been answered - the way we do it is to use a text block, the only text in the text block is 'Question Locked'. Then we select the publish text block as an image option in the text block properties and give the text block the name 'question lock'.We place this over the radio buttons of the question as Ben states and the 'Question locked' text gives the users the visual message that they can't reanswer the question.

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