If a question asks about nearly ninety-nine percent of dry air, which group is correct?

Correct answer: Nitrogen and oxygen

The correct answer is A: nitrogen and oxygen. Dry air means air considered without variable water vapour. Nitrogen makes up about 78 percent of dry air and oxygen about 21 percent, so together they account for approximately 99 percent. The figures are rounded, but the important idea is that these two gases dominate the composition of dry air. B is wrong because carbon dioxide and ozone occur in very small amounts. C is wrong because water vapour is excluded when we specify dry air, and dust is not a principal gaseous component. D is wrong because methane and neon are trace gases present in extremely small concentrations. A useful caution is that the composition of moist air changes when water vapour is included, but the standard “about 99 percent” statement refers to dry air. Memory cue: “78 plus 21 gives about 99: nitrogen plus oxygen.”