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
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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．that 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
In the text the author wants his marketers to understand that.
A．advertising to children must stop
B．a libertarian view in advertising is unethical
C．advertising to children must have a clear purpose
D．children must be treated differently when advertising
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