单项选择题
While I was in the university, I learned taking a photo,（）is very useful now for me.
A.it B.which C.that D.what
单项选择题 This kind of material expands（）the temperature increasing.
单项选择题 People at the party worried about him, because no one was aware（）he had gone.
单项选择题 A sudden noise of a fire-engine made him（）to the door.
