液压缸是利用油液的（）驱动机械对象实现（）运动的执行元件。
参考答案：压力能；直线往复
The first edition ________ in 1955 and ________ a best seller ever since. [ ]
A. was published; had been
B. was published; has been
C. has been published; has been;
D. had been published; had
值班员职责的首部由（）构成。
A、标题、文书编号
B、标题、文书编号、前言
C、文书编号、前言
D、标题、前言
