Correct Answer: Option B) 2 & 3
Statement 1: The composition of gases remains constant across all layers.This is incorrect.
The composition of atmospheric gases varies significantly with altitude. While the lower atmosphere (troposphere and stratosphere) maintains a relatively constant composition of major gases like nitrogen and oxygen, this changes dramatically at higher altitudes. In the thermosphere and exosphere, lighter gases like hydrogen and helium become more prominent, while heavier gases become scarce.
Statement 2: Oxygen becomes negligible at around 120 km altitude.This is correct.
At approximately 120 km altitude (in the thermosphere), oxygen molecules become extremely rare. The atmosphere at this height is so thin that oxygen concentration becomes negligible for practical purposes. This altitude marks the transition where atmospheric drag on satellites becomes minimal.
Statement 3: Hydrogen has the highest concentration among rare gases.This is correct.
Among the rare gases in the atmosphere, hydrogen indeed has the highest concentration. While hydrogen makes up only about 0.00005% of the atmosphere by volume, this is still higher than other rare gases like helium, neon, argon (though argon is sometimes classified separately), krypton, and xenon.
