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问题
单项选择题
以下哪项由中胚层分化而来（）
A.中枢神经系统
B.周围神经系统
C.表皮
D.真皮
E.呼吸上皮
答案
参考答案：
D
选择题
The little boy is dirty from head to foot because he ____ in the mud all morning.
A. has played
B. is playing
C. has been playing
D. was playing.
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问答题 简答题
何谓成分输血？
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