油船和化学品船舶的透气系统作用相似，是用来，（）
A、航行时保持舱内的压力
B、装货时排出舱内气体
C、卸货时防止舱内形成真空
参考答案：A, B, C
—What does your sister do ?
— . [ ]
A. She is in the supermarket.
B. She is very tall.
C. She is a teacher.
D. She is fine.
电机短时运行的标准持续时间分为（）四种。
A、10min，20min，40min和60min
B、10min，30min，60min和90min
C、10min，40min，80min和100min
D、10min，50min，70min和110min
