β-内酰胺类抗生素药物的基本性质是
A.分子结构中的共轭双键骨架,手性碳原子B.氢化噻唑环的羧基的酸性C.β-内酰胺环的不稳定性D.B+CE.A+B+C
参考答案:E
—May I go fishing with you,Dad?
—Of course .But you____finish your homework first. [ ]
A. mustn't
B. must
C. would
D. can't
机械密封的动环、静环、密封圈、弹性元件的工作状态如何?