单项选择题

A．They defend the bone against viruses.
B．They prevent oxygen from entering the bone.
C．They break down bone tissue.
D．They connect the bone to muscle tissue.
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单项选择题
A．Only one.B．Two.C．Three.D．More than three.
A．Only one.
B．Two.
C．Three.
D．More than three.
单项选择题
A．On short trips.B．On long trips.C．When flying over cities.D．When flying at high altitude.
A．On short trips.
B．On long trips.
C．When flying over cities.
D．When flying at high altitude.
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