Simon likes playing ________ football but Sandy likes playing ________ piano.
We had fun________ in the swimming pool last Sunday.
There must be a boy ________ basketball outside the door. I hear him crying.
Have you done ____ to improve the environment ?
—Who helped Betty tidy up the bedroom just now?—________. She cleaned it all by herself.
The workers in the Huashan Mountains have to ______rubbish to keep the mountains clean.