青霉素类的基本母核是：
A．7-氨基头孢烷酸
B．6-氨基青霉烷酸
C．β-内酰胺环
D．氨基苷结构
E．大环内酯结构
参考答案：B
－Why didn’t Susan come to see us yesterday as she promised?
－______.
A．She must be busy
B．She might have been free
C．She must have been busy
D．She can’t be free
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