单项选择题

What is one reason astronomers make long-time exposures

A．To spend less time at their telescopes.
B．To overcome the problem of weak light.
C．To take more photographs.
D．To photograph astronomical objects without using a telescop

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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, how can a speaker indicate that the topic is about to change

A．By pausing.
B．By raising pitch.
C．By lowering register.
D．By pointing to a chart.