In which atmospheric layer is air needed for human life most readily available?

Correct answer: Troposphere

Answer: A, the troposphere. The troposphere is the lowest atmospheric layer and lies directly above Earth’s surface, where people, animals, and plants live. Gravity causes most of the atmosphere’s mass to remain near the surface, so the troposphere contains relatively dense air with abundant oxygen and nitrogen for ordinary life. A is correct because this is the layer in which normal human breathing takes place. B, the mesosphere, is much higher and has very thin air. C, the thermosphere, contains extremely sparse gas and strong high-energy radiation, so it is not suitable for ordinary breathing. D, the exosphere is the outermost and most tenuous region, where particles are extremely far apart. These layers still contain gases, but not air readily usable by humans without life-support equipment. Memory cue: “troposphere = the layer where weather and everyday life occur.”