为什么在不得已时才选用下肢静脉？
参考答案：
因下肢浅层静脉常连接较深部的静脉，浅层静脉形成血栓时极易扩散至深部静脉，进而导致血栓的形成。
The tourists, had enjoyed themselves so much in the botanic garden, were worn out
A．all of what
B．all of which
C．all of them
D．all of whom
地电波方法涉及到以下（）的基本原理或方法的应用。
A.耦合电容分压
B.集肤效应
C.欧姆定律
D.电磁效应
