Which of the following two gases make up the bulk of the atmosphere?

Concept: The atmosphere is a mixture of gases in very unequal proportions by volume — a small number of major gases dominate the total volume, while every other gas is present only in trace amounts.

Application: By volume, dry air near Earth's surface is about 78% nitrogen (N2) and about 21% oxygen (O2). Together these two gases account for close to 99% of the atmosphere's total volume. The remaining roughly 1% is made up of trace gases such as argon, carbon dioxide, neon, helium, hydrogen, and ozone.

Cross-check: Checking each distractor pair against these known proportions confirms the choice — carbon dioxide (about 0.04%), hydrogen, helium, and ozone (each at parts-per-million to very small trace levels) never approach a combined share anywhere near 78%+21%. Only the nitrogen-oxygen pair accounts for the bulk of the atmosphere.