Some questions are automatically marked as "answered" when going to next page in one attempt, not matter they have not been introduced by the user at all.
1) restore the attached backup file.
2) Got to "Quiz One" and start one attempt.
3) Without introducing any response, press next.
4) Some questions appear as answered in the "summary box" upper left.
5) Press next again, the "Summary of attempt" is shown. It shows two answered questions and the user hasn't introduced them.
PS: Both questions seem to be truefalse ones