Problem with retention of multi choice question answers

I have a test with multi choice questions in it which works and scores perfectly. However, if the user doesn't get 100% we give them the opportunity to go back through the content and then re-take the test. Unfortunately because its the same set of questions the users originally selected answers are retained when the user comes back to the start of the test. Is there a way to clear all selected answers when the test is restarted? It is not a new session as the users will still be in the module and I don't want to create a new set of test questions if possible.

Thanks

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