轮船是采用“空心”的办法而浮在水面的，它所受的浮力________船的总重。轮船的排水量是指轮船满载时排开的水的_________。
等于；质量
在基地内部转线作业时，车辆装载货物高度不能超过距轨面（）mm。
A.4100
B.3200
C.2800
D.4400
How can you expect him to make any improvement ________ you never give him an opportunity to have a try?
A．unless
B．when
C．until
D．while
