【单选题】

Manners are important to happy relations among people.Everyone likes a person with good manners. No one likes a person with bad manners.But what are good manners How does one know what to do and what not to do

As a student, it is bad manners to come late to class. If you are late you should make an apology to the teacher at the time or after class. It is also bad manners to keep silent when the teacher asks you a question. If you do not know the answer, say "I don’t know" at once. If you do know, answer in a loud enough voice so that all the class may hear. It is polite for the students to help the teacher. Sometimes students can help their teacher to clean the blackboard, to close or open the door or windows. Sometimes there are papers to collect or to hand out. This kind of help is always appreciateD、
It is bad manners for you to say ______ when someone asks you a question.A．I don’t know
B．a lot of things
C．nothing