单项选择题
The sun heats the earth,（）is very important to living things.
A.that B.what C.which D.where
单项选择题 This is the problem（）we have no good solution as yet.
单项选择题 Last month, he paid a visit to the village（）he had once worked for five years.
单项选择题 Mr. Liu,（）at university in Changchun, works at Changhou Company now.
