A light year is the distance light travels in a year, about 9.5 trillion kilometers.
其中省略的关系代词是什么?这个关系代词在从句中充当什么成分?
A light year is the distance light travels in a year, about 9.5 trillion kilometers.
=A light year is the distance that /which light travels in a year, about 9.5 trillion kilometers.
【说明】这里的 travels 是及物动词,表示“移动”。关系代词 that (或which)在定语从句中作 travels 的宾语,所以可以省略。