单项选择题

Why do astronomers often use photographic plates

A．To decrease the time it takes to photograph objects.
B．To avoid using a telescope.
C．To sharpen the color of what they observe.
D．To obtain images of distant objects.

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单项选择题

What recommendation does the professor make about volume

A．Always use a microphone.
B．Avoid large rooms.
C．Never vary the volume.
D．Not to shout.

单项选择题

According to the professor, what can a speaker do to keep an audience's attention

A．Speak very loudly.
B．Ask questions frequently.
C．Vary tone, volume, and speed of speech.
D．Limit the speech to fifteen minutes.