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【多选题】
下列有关青霉素类和头孢菌素类抗生素结构的叙述正确的有A.
一个四元的β-内酰胺环，且均与一个五元环或六元环稠合
B.
β-内酰胺环是平面结构，但与稠合环不共平面
C.
C2位（即与N相邻的碳原子）连接一个羧基
D.
β-内酰胺环的α位连接一个酰胺侧链
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A.
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B.
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C.
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