With the nuclear crisis worsening in Iran, the world's attention is fixed again on     is called the Middle East.

A．which

B．that

C．what

D．it

（）算法抵抗频率分析攻击能力最强，而对已知明文攻击最弱。

A.仿射密码

B.维吉利亚密码

C.轮转密码

D.希尔密码