现场钻井队值班干部进入目的层作业每（）检查一次泥浆工的坐岗情况并签字。
A.6小时
B.4小时
C.2小时
D.1小时
参考答案：C
— Could I clean the bedroom for you?
— No, thanks. I it myself. [ ]
A. cleaned
B. have cleaned
C. was cleaning
D. had cleaned
氨基糖苷类抗生素的作用部位主要是在（）
A．细菌核蛋白体30S亚基
B．细菌核蛋白体50S亚基
C．细菌的细胞壁
D．细菌的细胞膜
E．DNA回旋酶
