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．A victim.B．A police detective.C．A robber.D．A pet bird.
A．A victim.
B．A police detective.
C．A robber.
D．A pet bird.
单项选择题
The problem of population is mainly man-made.
A．Y
B．N
C．NG
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