If dust particles are very low in a region, what difficulty may increase in cloud formation?

Correct answer: Lack of condensation nuclei

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.”