用所给词的适当形式填空
①—Where is Mr. Green?
—He ____ (go) to Beijing on business.
②You mustn't swim in the lake. Look at the sign “No ____ (swim). ”
③ After school, he always finishes ____ (do) his homework first.
④ My sister ____ (arrive) at the airport in two hours.
⑤ David is coming to visit us. I’m looking forward to ____ (meet) him.
