UPSC Prelims 2011— Question 74

Concept and answer: Option C is correct. CFLs use mercury vapour/phosphor while LEDs use semiconductors; LEDs are generally more efficient and longer-lived. The question should be approached by identifying the exact proposition being tested and then checking each alternative against that proposition. A familiar word or related fact is not enough; the option must be completely consistent with the underlying concept. Statement 1 is correct under the verified interpretation. Statement 2 is incorrect under the verified interpretation. Statement 3 is correct under the verified interpretation. Option A: Incorrect. It omits true statement(s) 3. Option B: Incorrect. It omits true statement(s) 1; includes false statement(s) 2. Option C: Correct. It selects exactly the valid statement combination and therefore matches the factual and logical requirements of the question. Option D: Incorrect. It includes false statement(s) 2. Exam takeaway: the decisive distinction is the one stated in the core explanation. Option C is the only choice that remains fully consistent with that distinction. This approach is useful for Prelims because many distractors contain a partially true statement but add an incorrect qualifier, reverse a relationship, confuse two institutions, or use an absolute term. Always test the entire option rather than recognizing only one familiar phrase.