单项选择题
The younger motorman as well as the others （） to operate the oil separator.
A.has learnt B.have learnt C.learn D.are learning
单项选择题 A water pump can not supply water after startingIt may be caused by （）.
单项选择题 Dress up the （）.
单项选择题 A mixed-flow pump is a （） pump classified by the working principle.
