Final answer:
The Earth's atmosphere contains enough sulfates and nitrates in some locations to create acid rain, due to the reaction of sulfur dioxide and nitrogen oxides with other chemicals.
Step-by-step explanation:
The correct answer to the student's question about the Earth's atmosphere is A. The atmosphere contains enough sulfates and nitrates in some locations to create acid rain. This occurs because sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which are primarily emitted from the combustion of fossil fuels, react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid (H₂SO₄) and nitric acid, resulting in acid rain. These acids can have harmful effects on soil, plants, aquatic ecosystems, and man-made structures.
As for the other options, B is incorrect; the atmosphere is not composed of 78% oxygen, rather it contains 78% nitrogen and 21% oxygen. C is partially correct in describing substances found in the atmosphere but incorrectly describes them as aurorae, which are actually a natural light display in the Earth's sky, typically in the polar regions, caused by the collision of solar wind and magnetospheric charged particles with the high altitude atmosphere. D is incorrect because the Earth's atmosphere decreases in density with increased elevation.