16.Listen to the passage and complete the following sentences(聽短文,完成下列內(nèi)容.每空格限填一詞)
21.If you do not understand the question,you should put up your hand and ask the teacher to _____ ______.
22.Another custom is that there is _____ ______for not doing your homework.
23.It is your responsibility to find out ___ ______ you have missed.
24.In America,it is thought _____ ______ be late,so you must be on time.
25.Entering a teacher's office or any other office with a closed door _____ _______first is not polite.