单项选择题

Why don’t all ice crystals in clouds immediately fall to Earth().

A.They are balanced by the pressure of rain droplets.
B.The effect of gravity at high altitude is random.
C.They are kept in the sky by air current.
D.They are moving about with fixed direction.

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单项选择题
Where are the photos much cheaper().

A. In a photographer’s studio.
B. In the library.
C. In the post office.
D. Not mentioned.
单项选择题
How many days will the tour guide stay in the hotel().

A.Two days.
B.No more than 3 days.
C.At least three days.
D.Less than one week.
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