UPSC Prelims 2015— Question 18

Correct answer: Option A. Fly ash is a fine residue produced by coal combustion in thermal power plants. It can be incorporated into bricks and can replace a portion of Portland cement in concrete, improving some material properties and reducing cement demand. It is not composed only of silicon dioxide and calcium oxide; its composition varies and may include other oxides and trace elements. Option-wise analysis: Option A: “1 and 2” is correct. It directly matches the factual or conceptual requirement tested by the question and is supported by the principle explained above. Option B: “2 only” is incorrect. It does not match the specific fact, definition, institutional role, geographical association, or constitutional/economic principle required here. The key distinction is the one identified in the core explanation, so this option can be eliminated. Option C: “1 and 3” is incorrect. It does not match the specific fact, definition, institutional role, geographical association, or constitutional/economic principle required here. The key distinction is the one identified in the core explanation, so this option can be eliminated. Option D: “3 only” is incorrect. It does not match the specific fact, definition, institutional role, geographical association, or constitutional/economic principle required here. The key distinction is the one identified in the core explanation, so this option can be eliminated. Prelims takeaway: Focus on the precise institutional role, constitutional provision, geographical association, or technical definition being tested. UPSC questions often place a broadly familiar fact beside a narrowly incorrect qualifier; identifying that qualifier is essential for elimination.