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
According to the text, the fundamental cause of the ice ages is ______.
A．periodic changes in the earth's orbit around the sun
B．fossil remains of one-celled animals
C．alternating cold and warm seasons
D．changes in distance of the earth from the sun
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
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