单项选择题

Carbon dioxide is now a threat to ______.

A．people staying indoors
B．people in poor health
C．the earth's vegetation
D．the earth's temperature
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单项选择题
People have different opinions about ______.
A．why the concentration of gas is rising steadily
B．to what degree the greenhouse effect can affect us
C．whether CO2 has anything to do with the greenhouse effect
D．what measures should be taken to lower the carbon dioxide levels
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.
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