Final answer:
Half of the atmosphere's mass is found below an altitude of 3 1/2 miles due to compression by the weight of the overlying atmosphere, with the correct answer being "3 1/2 miles, weight".
Step-by-step explanation:
The student's question pertains to the atmospheric pressure and the distribution of mass within the Earth's atmosphere. Atmospheric pressure is the force exerted per unit area by the weight of air above that point. The Earth's atmosphere is composed of several layers, and the mass of air is greater near the Earth's surface because the air is more compressed by the weight of the atmosphere above it. This compression is due to the force of gravity, which pulls the atmospheric gases towards the Earth's surface, creating higher density and pressure at lower altitudes.
The correct answer to the student's question is "c) 3 1/2 miles, weight", meaning that half of the mass of the atmosphere is found below an altitude of 3 1/2 miles because the air is compressed by the weight of the overlying atmosphere. The reference to weight is appropriate because it is the downward force exerted by gravity on the mass of the atmosphere.