— 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.

— Where is your father?— He ______ Beijing. He will be back in three days.

 But much of the world’s water ____ from rivers and lakes.

The boy followed the teacher and entered the classroom.

At last, she found her _______ pet dog.