单项选择题

A．They aren’t sure how these cells work.
B．They have difficulty identifying these cells.
C．They’ve found similar cells in other species.
D．They’ve learned how to reproduce these cells.
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单项选择题
A．It was more respected:B．More people used it.C．It was damaged by tourists again.D．It quickly got worse again after Lincoln’s death.
A．It was more respected:
B．More people used it.
C．It was damaged by tourists again.
D．It quickly got worse again after Lincoln’s death.
单项选择题
A．They break down bone tissue.B．They defend the bone against viruses.C．They connect the bone to muscle tissue.D．They prevent oxygen from entering the bone.
A．They break down bone tissue.
B．They defend the bone against viruses.
C．They connect the bone to muscle tissue.
D．They prevent oxygen from entering the bone.
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