Lettering and Numbering

Correct answer: B. When moist air cools to saturation and condensation occurs

Direct answer: B is correct. Rainfall needs atmospheric moisture and a process that cools moist air. When moist air rises or is cooled in another way, its temperature may reach the dew point. The air then becomes saturated, meaning it cannot retain all its water vapour in gaseous form at that temperature. Excess vapour condenses on tiny particles into cloud droplets or ice crystals. These particles may join, grow and eventually fall as precipitation. A is wrong because dry air lacks the moisture needed for rain. C is unrelated to ordinary rainfall formation. D is impossible in the stated context and gravity is important for droplets falling to Earth. Thus B correctly describes saturation followed by condensation. Memory cue: “Moisture plus cooling leads to clouds and possible rain.”

Correct answer: B. Both can act as condensation nuclei

Direct answer: B is correct. Atmospheric dust and sea-salt particles are aerosols, meaning tiny solid or liquid particles suspended in air. When moist air cools to saturation, water vapour can collect on suitable aerosol surfaces. Such particles are called condensation nuclei and help cloud droplets form. Sea-salt particles may enter the air from breaking waves and sea spray, while dust can be lifted from dry land, but both can serve a similar role in cloud formation. A is wrong because these particles do not come only from fireballs. C is wrong because they do not convert oxygen into nitrogen. D is wrong because both can enter and remain suspended in the atmosphere. Thus B identifies their important similarity. A useful caution: condensation nuclei help condensation, but their presence alone does not guarantee rainfall.