车轴齿轮箱内装有（）油泵，在机车正反向运行时，都能供给润滑油。
A、单向齿轮
B、双向齿轮
C、定向齿轮
D、定向柱塞
参考答案：B
The TV play _____ for ten minutes when I turned on the TV. [ ]
A. had been on
B. had started
C. had begun
D. has been on
应急发电机、照明、蓄电池和开关失灵可导致船舶滞留。
