CONCEPT: - Length is a measure of the size or distance between two points. The SI unit of length is the meter (m). - Light year: It is the distance travelled by light in one year. - A light-year is a convenient unit of measurement of large distances. - Generally, a light-year is used to represent astronomical distances. - One light-year is approximately 9 trillion (9×1012)km. EXPLANATION: - A light-year is a unit of length. Therefore option 4 is correct. - It is the distancetravelled by light in a vacuum in one year. - A light-year is used in astronomy to measure the distances of nearby stars. ⇒1 light-year =11y=9.467×1015m
