Final answer:
Sit-ups are the only muscle-strengthening activity that can be done in school to enhance muscle strength, mass, and fitness.
Explanation:
Sit-ups is the only muscle-strengthening activity that can be completed in school. Muscle-strengthening activities, such as lifting weights, resistance bands, and bodyweight exercises like push-ups, fall under this category. These activities help improve muscle strength, mass, and overall fitness.
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