Now most families have only one child _____ our country’s one-child policy.
helps build a better world for everyone, especially children around the world.
—People have realized how important the environment is, but still trees ____in the world every year.— We must do something to stop it.
Do you know _____?
There is _____ knocking at the door. Go and see who it is.
—What was your hometown like five years ago?—It _____ very old and dirty.