Meaning & Definition

The governing idea is that abundance and biological importance are different measures. Oxygen is directly used by most aerobic organisms in cellular respiration, where food molecules are broken down to release usable energy. Nitrogen is more abundant but is not directly used as the main respiratory gas by humans and most animals. Therefore option A is correct; the other choices misidentify oxygen as inert, dust, or salt.

Dry air is a useful reference because water vapour is highly variable in space and time, whereas the principal permanent gases have comparatively stable proportions. By excluding water vapour from the calculation, scientists can compare the relative percentages of nitrogen, oxygen, argon, and other permanent gases more consistently. Hence option A is correct. Dry air still contains gases, including nitrogen and oxygen; it is not oxygen alone or an empty mixture.

Direct answer: A, lack of condensation nuclei. Clouds form when moist air cools and water vapour changes into tiny liquid droplets or ice crystals. In ordinary atmospheric conditions, these droplets often begin forming on very small surfaces supplied by dust, sea salt, smoke, or other aerosol particles. These surfaces are called condensation nuclei. If dust and similar particles are scarce, water vapour may have greater difficulty starting condensation, even when the air is sufficiently moist and cool. Option B is wrong because low dust does not remove atmospheric nitrogen. Option C is wrong because oxygen does not become solid merely because dust is scarce; extremely low temperatures would be needed. Option D is wrong because argon is a largely inert gas and does not normally burn. Memory cue: dust gives water vapour a surface on which to gather—“no nuclei, harder to begin droplets.”