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GS Paper IUPSC Prelims 2021
Space & ISRO
Question No. 103Interactive Test Mode
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Q103. Which one of the following is a reason why astronomical distances are measured in light-years?

Which one of the following is a reason why astronomical distances are measured in light-years?

Select the correct option below (or cross out options to eliminate)
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Official Answer Key & Detailed Solution

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Official UPSC Answer KeyUPSC Prelims 2021 • Science and Technology

Correct Answer: Option (D) — The speed of light is always the same.

Concept & Option Breakdown

Standard Reference Verified

A light-year is defined as the distance light travels in a vacuum in one Julian year (approx. 9.46 trillion km). Because the speed of light in a vacuum ($c \approx 3 \times 10^8\text{ m/s}$) is a fundamental universal constant that remains invariant across all reference frames, it provides a reliable, standard yardstick for astronomical distances.

Elimination Heuristic & Exam Approach

Look for the fundamental physical constant: light travels at a constant speed in a vacuum regardless of the motion of the source or observer.

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Question Specifications & Syllabus Mapping

Examination & YearUPSC Civil Services Preliminary Examination 2021 (Q103)
Paper & Marking SchemeGS Paper I • 2 Marks (-0.66 Negative for incorrect response)
Syllabus Subject & TopicScience and Technology → Space & ISRO
Official Answer KeyOption (D) — The speed of light is always the same.

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Frequently Asked Questions — UPSC Prelims 2021 Q103

What is the official answer for UPSC Prelims 2021 GS Paper I Question 103 (Science and Technology)?

The official UPSC answer key for this 2021 Science and Technology (Space & ISRO) question is Option (D): "The speed of light is always the same.". Correct attempts carry 2 Marks with -0.66 Negative penalty for wrong answers.

Which UPSC Prelims syllabus topic does this 2021 question belong to?

This question belongs to GS Paper I under the subject "Science and Technology" and micro-topic "Space & ISRO". You can explore all repeated questions from Space & ISRO in Acme AI's 3D Prelims PYQ Realm or practice 50 full-length and sectional tests in the APT Prelims Test Series.

How can I solve this Space & ISRO UPSC Prelims question using option elimination?

Look for the fundamental physical constant: light travels at a constant speed in a vacuum regardless of the motion of the source or observer.

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