软交换满配置时需要（）个机架。
A.3
B.5
C.7
D.9
参考答案：C
—Excuse me, must I finish my homework now?
—No, you _______. You can hand it in tomorrow. [ ]
A. mustn't
B. needn't
C. shouldn't
D. wouldn't
酶的竞争性抑制剂具有下列哪种动力学效应？（）
A.Vmax不变，Km增大
B.Vmax不变，Km减小
C.Vmax增大，Km不变
D.Vmax减小，Km不变
