UPSC Prelims 2021— Question 26

A light-year is a unit of distance defined by the distance light travels in one year. Its use for astronomical distances is convenient because the speed of light in vacuum is a well-defined constant, making the unit reproducible. Stellar distances do change over time, gravity is irrelevant to the definition, and the statement about straight-line travel is not the reason for the unit. The final answer-key position for Set C identifies option D as the correct option. Option A — Distances among stellar bodies do not change. — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. Option B — Gravity of stellar bodies does not change. — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. Option C — Light always travels in straight line. — is not correct because it does not satisfy the specific condition tested in the question; it represents an alternative that is contradicted, incomplete, or less precise than the accepted answer. Option D — Speed of light is always same. — is correct because it is the alternative that matches the constitutional, historical, geographical, scientific or economic principle established above. For PYQ interpretation, the distinction between the correct alternative and merely related facts is important. UPSC often places plausible distractors beside the precise statement that matches the underlying rule, chronology, definition or empirical relationship. The answer should therefore be read against the exact wording of the question rather than against a broadly related fact.