Which one of the following is a reason why astronomical distances are measured in light-years?
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Answer: D. A light-year is the distance light travels in one year. It works as a unit because the speed of light in vacuum is constant (about 3 lakh km per second), so the distance it covers in a year is fixed.
Watch for: A unit must be fixed; that is why the constant speed of light is the reason.
Why the other options are wrong
- A. Wrong because distances between stars do change over time.
- B. Wrong because gravity has nothing to do with the unit.
- C. Wrong because light can bend near massive objects, and that is not the reason.
Source to check: NCERT Class 11 Physics Part 1, Chapter 2 Units and Measurement
Answer from the official UPSC key. Written and fact-checked by AI against the official key. Spotted a mistake? Report it.