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.
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单项选择题
A．They carried passengers, but not cargo.B．They were large, but surprisingly fast.C．They were the first successful steam-powered ships.D．They were more reliable than other ships of the 1860’s.
A．They carried passengers, but not cargo.
B．They were large, but surprisingly fast.
C．They were the first successful steam-powered ships.
D．They were more reliable than other ships of the 1860’s.
单项选择题
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.
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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