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As the title described, I have turned off the "no answer" option in the presentation setting but the "no answer" option is still there in the preview. What should I do? 

Please answer the questions about version, hosting, etc 

Sorry for opening this issue up, but I seem to encounter the same issue:
In my "Presentation" settings I set "show "no answer" to "off", but in my questions the "no answer" option (for non-mandatory questions) is still shown.

Version: 5.2.6+211213 Community Edition (self-hosted)
Theme: custom fruity variation

Please provide a small sample survey (.lss file).

Thank you for the quick response:
Below a survey with one (Yes/No) question.
Maybe I misunderstand the survey options. So let me quickly explain how I understand the settings:
I know that LS automatically inserts a "no answer" option when you set the question to "not-mandatory". In the past I always made the questions mandatory and just inserted my own "no answer" option. However, I find it more elegant to signal to the participant that some questions are mandatory and some are not. 
Anyway, I thought the settings in the "Presentation" section can change the behavior of non-mandatory questions "by setting the show "no-answer" to "on" or "off". However, in my survey (and in the attached sample survey) setting "show "no-answer"" to "off" does not seem to change anything for the behavior of the question.