Humidity, evaporation, and condensation

Correct answer: A. H₂O

H₂O is the chemical formula for water, consisting of two hydrogen atoms bonded to one oxygen atom. When water exists in the gaseous state, it is called water vapour. Water vapour is naturally present in the atmosphere and is also an important greenhouse gas because it absorbs and re-emits infrared radiation. CO₂ is carbon dioxide, O₃ is ozone, and N₂ is nitrogen, so H₂O is the only correct answer.

Correct answer: B. 50%

Relative humidity is calculated as RH = (actual vapour pressure ÷ saturation vapour pressure) × 100. Substituting the given values gives RH = (12 ÷ 24) × 100 = 0.5 × 100 = 50%. Thus option B is correct. The units cancel because both pressures are measured in the same unit.

Correct answer: A. Water pollution will increase

The governing concept is water pollution caused by the discharge of harmful substances into a water body. Untreated industrial effluent may contain toxic chemicals, heavy metals, suspended solids, acids, or organic matter. After entering a river, these substances can lower water quality, harm aquatic plants and animals, and sometimes reduce dissolved oxygen when microorganisms decompose organic pollutants. Therefore, option A is correct because untreated discharge increases water pollution. Option B is opposite to the expected effect. Option C is also incorrect because pollution often reduces, rather than increases, oxygen available to aquatic organisms. Option D is unrelated to waste treatment and river pollution.