单项选择题

Questions 19 to 22 are based on the conversation you have just heard.

A．A fuel that burns cleanly.
B．An oil additive that helps cool engines.
C．A material from which filters are made.
D．An insulating material sprayed on engine parts.
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单项选择题
A．It influences the surface temperature of Mars.B．It protects living beings from harmful rays.C．It keeps a planet from overheating.D．It is the main component of the air people breathe.
A．It influences the surface temperature of Mars.
B．It protects living beings from harmful rays.
C．It keeps a planet from overheating.
D．It is the main component of the air people breathe.
单项选择题
A．Because every species becomes extinct.B．Because humans beings are powerful enough to kill other species.C．Because of over-population.D．Because human being is still a young species though it is exploiting the environment.
A．Because every species becomes extinct.
B．Because humans beings are powerful enough to kill other species.
C．Because of over-population.
D．Because human being is still a young species though it is exploiting the environment.
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