Undesired "previous" and "next" buttons visible in interactive questions

Dear community,

I'm working on an e-module and I simply don't know why the 'previous and 'next' buttons appear. To be more specific, they appear when you answer a multiple-choice question, get the feedback, then continue to the next slide. The problem start when you go back to that multiple-choice question. All of a sudden, these buttons have appeared.

I've tried many things and scanned the internet for help, without result.. Could anyone have a look? I've uploaded the 'story' file so you can have look yourself. Slide 2/18 is the first multiple-choice question, so you can see it for yourself there ;-)

Thanks in advance!