单项选择题


A．unless
B．whether
C．as
D．though
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单项选择题
What is the specific feature of fossil fuels
A．They are colorless.
B．They are odorless.
C．They are rich in carbon.
D．They are easy to freeze.
单项选择题
Why is more earth's heat blocked from escaping with more carbon dioxide in the atmosphere
A．Carbon dioxide prevents the sun's energy from going back into space.
B．Carbon dioxide radiates the sun's energy onto the earth.
C．Carbon dioxide produces more heat in the atmosphere.
D．Carbon dioxide takes in the sun's energy at infra-red wavelengths.
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