单项选择题

Air pollution, together with overpopulation, ____ many problems in big cities.

A．are causing
B．is causing
C．are caused
D．is caused
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单项选择题
She has no idea of what the book is about. She ____ have read it carefully.
A．mustn’t
B．can’t
C．shouldn’t
D．needn’t
单项选择题
____ from space, our earth, with water covering 70% of its surface, appears as a blue planet .
A．Seeing
B．To be seen
C．Seen
D．Having seen
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