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．Its chemical elements must be studied.B．Its temperature must be lowered.C．Big spaceships must be built.D．Its atmosphere must be changed.

A．Its chemical elements must be studied.
B．Its temperature must be lowered.
C．Big spaceships must be built.
D．Its atmosphere must be changed.

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Questions 19 to 22 are based on the conversation you have just heard. 您的浏览器不支持 audio 元素。

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