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

It can be concluded from the passage that.

A．the circulation of atmosphere protects the biosphere and makes life on earth possible.
B．if the protective atmosphere is too damaged by human activity, all life will cease.
C．life is the result of complex interdependent events of nature
D．despite the complexity and power of the atmosphere humans can affect it greatly

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．demonstrates climatic changes prior to humans