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

1. A light-year is a unit of distance, defined as the distance light travels in a vacuum in one Julian year. 2. Astronomical distances are vast, making conventional units like kilometers or miles impractical. 3. The reason for using the light-year is that the speed of light in a vacuum (denoted by 'c') is a fundamental physical constant and is always the same (approximately 299,792 kilometers per second). 4. This constant speed provides a reliable and universally understood basis for measuring the immense distances to stars and galaxies. 5. Option (A) is incorrect; distances among stellar bodies do change due to their motion. 6. Option (B) is incorrect; the gravity of stellar bodies can change (e.g., if they lose mass) and is not the basis for the distance unit. 7. Option (C) is incorrect; while light travels in a straight line in uniform space, its path can be bent by gravity (gravitational lensing). However, the constancy of its speed is the key factor for its use in defining distance.