单项选择题

A．On short trips.
B．On long trips.
C．When flying over cities.
D．When flying at high altitude.
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单项选择题
A．They defend the bone against viruses.B．They break down bone tissue.C．They prevent oxygen from entering the bone. D) They connect the bone to muscle tissue.
A．They defend the bone against viruses.
B．They break down bone tissue.
C．They prevent oxygen from entering the bone. D) They connect the bone to muscle tissue.
单项选择题
A．People’s health.B．People’s lives and futures.C．The end of the world.D．The Great Depression of 1929.
A．People’s health.
B．People’s lives and futures.
C．The end of the world.
D．The Great Depression of 1929.
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