He helps them by giving them some food(对划线部分提问) ______ ______he________them?

He helps them by giving them some food(对划线部分提问) ______ ______he________them?
You mustn‘t sing in the classroom。改为祈使句
____ _____in the classroom.
Trees can clean the air.同义句转换
Trees can _____the air______.