单项选择题

The first paragraph of the text deals mainly with the earth atmosphere's ______.

A．sheltering effect
B．reviving effect
C．invigorating effect
D．cleansing effect
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单项选择题
The best definition of biosphere as it is used in the text is ______.
A．the protective envelope formed by the atmosphere around the living earth
B．part of the earth and its atmosphere in which life can exist
C．the living things on earth whose existence is made possible by circulation of the atmosphere
D．the circulation of the atmosphere's contribution to life on earth
单项选择题
According to the text, fossil studies of radiolaria are significant because they ______.
A．live in the depths of the ocean
B．are one-celled
C．have survived through the ages
D．demonstrate climatic changes prior to humans
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