Which sentence uses fall behind to mean fail to keep pace with work?
Sam's old folder fell apart in his bag.
Sam fell out with a classmate after an argument.
After missing two classes, Sam fell behind with his assignments.✓Bonne réponse
Sam fell over while carrying his books upstairs.
Explication
Falling behind means failing to keep pace, which follows naturally from Sam missing classes and accumulating unfinished work. Falling out would describe a quarrel, not a growing delay in assignments.