A、Always use a microphone. B、Avoid large rooms. C、Never vary the volume. D、Not to shout. D在谈到音量时，教授建议演讲者不要依赖麦克风，因为一个优秀的演讲者是可以做到

Listen to part of a lecture in speech class.
    Today we’re going to practice evaluating the main tool used when addressing groups--the voice. There are three main elements that combine to create either a positive or negative experience for listeners. They can result in a voice that is pleasing to listen to and can be used effectively. Or they can create a voice that doesn’t hold attention, or even worse, causes an adverse reaction.
    The three elements are volume, pitch, and pace. When evaluating volume keep in mind that a good speaker will adjust to the size of both the room and the audience. Of course, with an amplifying device like a microphone, the speaker can use a natural tone. But speakers should not be dependent on microphones--a good speaker can speak loudly without shouting.
    The second element, pitch, is related to the highness or lowness of the sounds. High pitches are, for most people, more difficult to listen to, so in general speakers should use the lower registers of their voice. During a presentation, it’s important to vary pitch to some extent in order to maintain interest.
    The third element, pace, that is how fast or slow words and sounds are articulated, should also be varied. A slower pace can be used to emphasize important points. Note that the time spent not speaking can be meaningful,  too.  Pauses ought to be used to signal transitions or create anticipation. Because a pause gives the listener time to think about what was just said--or even to predict what might come next--it can be very effective when moving from one topic to another.
    What I’d like you to do now is watch and listen to a videotape and use the forms I gave you to rate the speaking voices you hear. Then tonight I want you to go home and read a passage into a tape recorder and evaluate your own voice.
21. What is the main point the professor makes?
22.According to the professor, what can a speaker do to keep an audience’s attention?
23.What recommendation does the professor make about volume?
24.According to the professor, how can a speaker indicate that the topic is about to change?
25.What are the students going to use a tape recorder for?