To be a good teacher, you need some of the _ of a good actor; you must be able to hold the attention and interest of your audience; you must be a clear speaker, with a good, strong, pleasant voice which is fully under your control; and you must be able to act what you are teaching, in order to make its meaning clear. Watch a good teacher, and you will see that he doesn't sit still before his class; he stands the whole time he is teaching; he walks about, using his arms, hands and fingers to help him in his explanations, and his face to express feelings. There are very important differences between the teacher's work and the actor's. The actor has to speak words which he has learnt by heart; he has to repeat exactly the same words each time he plays a certain part, even his movements and the ways in which he uses his voice are usually fixed beforehand .What he has to do is to make all these carefully learnt words and actions seem natural on the stage. A good teacher works in quite a different way. His audience take an active part in his play: they ask and answer questions, they obey orders, and if they don't understand anything, they say so. The teacher can't learn his part by heart, but must invent it as he goes along. I have known many teachers who are fine actors in class but are unable to take part in a stage-play because they can't keep strictly to what another has written. From the passage, we know  _  .
a good teacher should be a good actor a good teacher may be unable to act well on the stage a good actor should be a good teacher a good actor may act well in class