He could play the piano at the age of five.
The rubbish _____ is produced every day is becoming a serious problem in cities around the world.
More and more trees were cut down. _____, many animals are dying out.
We should often keep _____ touch with each other, because we are good friends.
—What do you think this coat is made _____? —Wool. It’s used for _____.
—Do you know _____? —Sorry, I don’t know.