Complete the professor's reassuring email to a highly caffeinated and stressed-out freshman.

"Please get some sleep! The supplementary reading on the syllabus is purely for fun, so you ______ read chapters 4 and 5 for tomorrow's class."

The correct answer is don't have to.

We use don't have to to show a lack of obligation. It means the action is not necessary, but it isn't forbidden either (the student can read the chapters if they want to, but they aren't required to).

Using mustn't would mean the professor is forbidding the student from doing the reading!