X线质的表示方法主要有（）
A.半价层
B.管电压
C.管电流
D.管电流量
E.阳极靶面物质原子序数
参考答案：A, B
_____ foreigners come to China to visit the Great Wall every year. [ ]
A. Thousand of
B. Thousands of
C. Two thousands
D. Two thousand of
元中叶时，海外贸易达到了空前的繁荣。
