Boyle's Law for constant temperature

Steps:

• Initial length L1 = 20 cm, so initial volume V1 ∝ 20 cm.
• New length L2 = 20 cm + 40 cm = 60 cm, so new volume V2 ∝ 60 cm.
• V2 = 3 V1, since cross-sectional area is constant.
• By Boyle's law (P1 V1 = P2 V2), P2 = P1 V1 / V2 = p / 3.

Why B is correct:

• Boyle's law states that pressure is inversely proportional to volume at constant temperature, so tripling volume divides pressure by 3.

Why the others are wrong:

• A: Halving pressure assumes doubling volume, but volume triples.
• C: Doubling pressure assumes halving volume, opposite of what happens.
• D: Tripling pressure assumes dividing volume by 3, but volume increases.

Final answer: B