20. --- I think the waiter has forgotten us. We ________ here for over half an hour!

　 --- I think you’re right. He probably thinks we have already ordered.

　　　 A. are waiting　　　　　　　　　　　　　　 　 　　　　 B. have been waiting

　　　 C. have waited　　　　　　　　　　　　　　　　　　　　　 　 　 D. will wait