UPSC Prelims 2018— Question 57

**Concept and analysis:** Earth's magnetic field has undergone many reversals over geological time, with intervals commonly measured in hundreds of thousands of years, though the timing is irregular. The early Earth did not have an atmosphere containing 54% oxygen; free oxygen accumulated much later through biological processes. Living organisms, especially photosynthetic organisms, profoundly altered atmospheric composition, so statements 1 and 3 are correct. The question is testing precise UPSC-level identification rather than a broad association. The key is to distinguish the exact institutional, constitutional, historical, geographical, economic or scientific feature being asked. **Option-wise evaluation** **Option A:** Incorrect. It selects 1 only, but that combination does not match the verified statement-level assessment; it either includes a false statement or omits a true one. **Option B:** Incorrect. It selects 2 and 3 only, but that combination does not match the verified statement-level assessment; it either includes a false statement or omits a true one. **Option C:** Correct. It selects statements 1, 3; these are the statements supported by the relevant constitutional, economic, historical, scientific or geographical facts. **Option D:** Incorrect. It selects 1, 2 and 3, but that combination does not match the verified statement-level assessment; it either includes a false statement or omits a true one. **UPSC takeaway:** The safest way to solve this type of question is to anchor the answer in the defining feature of the concept and then test every option against that feature. Closely related institutions or terms are often used as distractors. In statement-based questions, do not judge the option as a whole before checking each statement independently. In factual questions, compare the wording of each option with the exact definition, location, institutional role, chronology or scientific property. The correct answer follows from that verification rather than from familiarity with a keyword.