Atmospheric pressure decreases with altitude in a mercury barometer

Steps:

• A sealed-end manometer functions as a barometer, where height h of the mercury column balances atmospheric pressure.
• Atmospheric pressure acts on the open end, pushing mercury up the sealed tube against vacuum.
• At higher altitudes, air density drops, reducing atmospheric pressure.
• Lower pressure supports a shorter mercury column, decreasing h.

Why A is correct:

• In a barometer, h is directly proportional to atmospheric pressure (P = ρgh, where ρ is mercury density and g is gravity); decreasing P reduces h.

Why the others are wrong:

• B: Atmospheric pressure decreases, not increases, with height.
• C: Atmospheric pressure decreases, not increases, with height.
• D: Decreasing pressure causes h to decrease, not increase.

Final answer: A