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.
填空题
B
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