在一张比例尺为1︰50000的地图中,小明家到动车站的距离有0．2米,则小明家到

关于参考系的选择，以下说法中正确的是（　　）

A．参考系都是看成假定不动的物体

B．不是任何物体都可以被选作参考系

C．参考系必须选择静止不动的物体

D．参考系必须是和地面连在一起的物体

简述检漏的操作方法。

—Did you meet Tom at the airport?

—No. His plane ________ by the time I ________ there. [ ]

A. has taken off; got

B. took off; had got

C. took off; arrived

D. had taken off; arrived

已知某汉字的区位码是3222，则其国标码是( )。

A．4252D

B．5242H

C．4036H

D．5524H

非系统风险