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 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

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.