As shown in the figure, ③ is the fulcrum o,

The movements of hands and feet make the lever rotate clockwise around the fulcrum O, and the position where people sit prevents the lever from rotating clockwise around the fulcrum O, so both hands and feet are power, and the force exerted on the lever by sitting posture is resistance.

Under the same conditions, other conditions remain unchanged, the hand in position ① is longer than the power arm in position ②, which is more labor-saving.

So choose D.