离心泵吸入管路底阀的作用是()。
A. 阻拦液体中的固体颗粒
B. 防止启动前充入的液体从泵内漏出
C. 避免出现气蚀现象
D. 维持最低的允许吸上高度
There _____ two cups of tea on the table.A、isB、areC、was
A.is
B.are
C.was
——“Who are you going to play ()? ”—— “Grade two”.A、aboutB、withC、against
A.about
B.with
C.against
The students put down their pens when the teacher _______ them to stop writing.A、bega
A.began
B.told
C.walked