单项选择题

Questions 19 to 22 are based on the conversation you have just heard.

A．They defend the bone against viruses.
B．They prevent oxygen from entering the bone.
C．They connect the bone to muscle tissue.
D．They break down bone tissue.

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单项选择题

A．Possibly not.B．Yes, of course.C．Definitely not.D．Not mentioned.

A．Possibly not.
B．Yes, of course.
C．Definitely not.
D．Not mentioned.

填空题

Questions 19 to 22 are based on the conversation you have just heard. 您的浏览器不支持 audio 元素。

Questions 19 to 22 are based on the conversation you have just heard.