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Your answer is fully submitted after hitting the blue 'submit answer' button. You will see 'Your response was received' confirming your answer is recorded:
You cannot CAN now change your answer for this question once clicking submit answer/receiving this confirmation message! To do so, simply click the new response you'd like to enter and click submit answer again.
Notice that the ability to choose the answer is gone from the current answer that has been submitted. The last answer submitted will be the one graded; any previously submitted answers will not be scored.
On the last page, results, you will be alerted if answers are needed for any problems.
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For this reason we recommend selecting and submitting answers in succession when you are ready to commit to your response, and reserving time at the end of the exam to review your submissions as time allows.