The best answer is B. push-ups.
Explanation:
1. Push-ups require the action of a first-class lever in the body.
2. In a push-up, the fulcrum is at the elbow joint, the effort (force applied) is at the hands pressing down against the ground, and the load (body weight) acts on the trunk and legs.
3. The effort arm (distance between the fulcrum and the point where the force is applied) and the load arm (distance between the fulcrum and the point where the load acts) play a role in the mechanical advantage of the lever.
4. By performing a push-up, you are utilizing your body as a first-class lever to raise and lower yourself, engaging various muscles including the chest, shoulders, triceps, and core.
Therefore, among the options provided, push-ups involve the action of a first-class lever in the body.
