I think it's essential to have another response status such as "unchanged" or "answer not changed".
While I understand your scenario where the student re-attempts a long quiz, goes away and comes back some time later and loose his/her place, I'll tell you that's been posed in my organization.
We have recurrent training for safety, occurance reports, policies, etc. We have a lesson and then of course a quiz afterwards. Students must get 100% to pass. After the quiz, students review their results and re-attempt the quiz and re-attempt only the answers they have incorrect. This repeats until they get 100% (in case you are wondering, this is more for the student to understand and agree to company procedures)
When a student re-attempts 2 incorrect responses on a 15 question quiz, and sees 13 "not answered" statuses, that can confuse many students