In the past, ice used to ______ used to ______ the air in a room.

Before I talked with ______ scientist, I had never met______ clever person like him.

Dad, _____ is my friend , Xiao Hua. –Hello, Xiao Hua.

_____ are your trousers? — Black. [ ]

We were about to go outing______ it began to snow heavily.