Newton's Third Law: Action-Reaction Pairs

Steps:

• Newton's third law states that forces between two interacting bodies are equal in magnitude and opposite in direction.
• Identify P as the gravitational force exerted by Earth on Moon.
• The reaction to P is the force exerted by Moon on Earth, which is R.
• Confirm Q and S involve the Sun but act on different bodies, so their reactions are not listed.

Why B is correct:

• P and R are the equal and opposite gravitational forces between Earth and Moon, directly following Newton's third law (F_{EM} = -F_{ME}).

Why the others are wrong:

• A: P and Q act between different pairs (Earth-Moon vs. Sun-Earth).
• C: Q and S both originate from the Sun but on separate bodies (Earth and Moon).
• D: R and S involve distinct interactions (Earth-Moon vs. Sun-Moon).

Final answer: B