The teacher was surprised to see lots of people_____at the back of the room _____worried. [ ]

A.seating;looked

B.seated;looking

C.sitting;looked

D.sat;looked

按密钥的使用个数，密码系统可以分为______。

A．置换密码系统和易位密码系统

B．分组密码系统和序列密码系统

C．对称密码系统和非对称密码系统

D．密码学系统和密码分析学系统