There is no correct answer (though there could be incorrect answers). Perhaps you want to investigate why the 30 students didn't submit the questionnaire. In t

There is no correct answer (though there could be incorrect answers). Perhaps you want to investigate why the 30 students didn't submit the questionnaire. In the name of the study's sanctity, you'd best convince them to complete it. But you must not coerce them, or else the results would be tainted. So you invent a time machine... OK, I'm done.