Before your time has expired on tests, you are now able to change your responses even Answers may now be changed (after you've submitted them) until you have completed your assessment (ie all answers are in). Keep this in mind as you go through your exam–once you answer all the questions your exam will close to you, so reserve one question to not answer until you are certain you are ready to submit your test.
To normally submit your response on an exam, select your chosen answer and click the blue 'submit answer' button. You will see 'Your response was received' confirming your answer is recorded, with your selected response no longer able to be selected:
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