X原子的最外层上有2个电子，元素Y的化合价为-1价，由X、Y两元素形成的化合物的

Not until the train pulled into the next station ____ her suitcase gone.

A．Mary had found

B．had Mary found

C．did Mary find

D．Mary found

下面对常模的理解有误的是（）

A、常模样本需要有代表性

B、常模样本不宜太小

C、常模样本需要及时修订

D、常模样本可随意选取

一只汽油桶可装45L汽油，我们说它的什么是45L？[ ]

A．体积    

B．容积    

C．质量

– Mary was crying sadly.

-- Oh, she ________ the bad news that her mother was badly injured in the accident.

A．must know

B．might have known

C．must have known

D．should have known

高频闭锁距离保护的缺点是：当线路上有（）时，高频闭锁距离保护会误动或拒动；在（）断线时，保护可能会误动。