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

1. A

   Distances among stellar bodies do not change.

   Distances among stars do not change. They do, slowly.

2. B

   Gravity of stellar bodies does not change.

   Gravity does not change. Irrelevant to the unit.

3. C

   Light always travels in straight line.

   Light always in a straight line. Curved by gravity; not the reason for the unit.

4. D

   Speed of light is always same.

   Speed of light is always the same. A constant c makes ‘how far light travels in a year’ a stable yardstick.

Summary. Official key is (d). A light-year is distance = c × one year. It is useful because the speed of light in vacuum is invariant. Stellar distances, gravity and straight-line travel are not that reason.