People usually work hard ____ the day, and they have a good rest ____ night.[ ]
Chinese people have been using paper for over______ years.
How will you help with the activity?— We will ______ leaflets to ask people to take part.
About two _________boys took part in the running race.
Air quality in our city is becoming worse and worse. – Yes. These days’ PM2.5 readings are frightening. We must do something to _______ air pollution.