一道高中宾语从句？

If he had a real talent for painting,a painter he should be；if not ,he must take to some other craft,____he would have the chance of making himself a decent livelihood.
为什么用where 不用which？

这是非限定性定语从句，where作状语修饰。。。craft,而which不能。

sunlight    is    a kind of energy_that is used to benefit human beings

这是定语从句  关系代词that  代替先行词 energy 在定从中 作主语 不能省略

sunlight  是主语  is是系动词   a kind of energy是表语

_that is used to benefit human beings是定语