1.- You must turn in all your work on time.
Well done, this is the correct answer due to the modal that expresses something necessary.
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2.- You can't afford to lose the semester.
Great job, the answer is correct because can’t express negative conclusions.
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3.- You should arrive early and attend all classes.
Great job. An obligation as a student is to go to class and arrive early as you should.
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4.- You could gather students in the same situation to help each other.
Well done, could will always refer to possibilities.
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5.- You have to graduate on time with everyone else.
Well done. As a student you have obligations to complete.
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