Boyle's Law for constant temperature

Steps:

• Boyle's Law states P1 V1 = P2 V2 at constant temperature.
• Volume is proportional to column length since cross-sectional area is constant.
• Initial volume V1 ∝ 20 cm; final volume V2 ∝ 50 cm, so V2 = 2.5 V1.
• P2 = (V1 / V2) P1 = (20/50) p = 0.4p.

Why A is correct:

• Boyle's Law requires pressure to be inversely proportional to volume, yielding 0.4p for a 2.5-fold volume increase.

Why the others are wrong:

• B: Assumes volume increase by only 1.25 times, not 2.5.
• C: Applies to volume decrease, not increase.
• D: Assumes direct proportionality, violating Boyle's Law.

Final answer: A