典型的杀毒软件有（）
A.瑞星
B.CiscoWorks
C.HPOpenView
D.金山毒霸
参考答案：A, D
I saw him _______ out of the room. [ ]
A. go
B. had gone
C. has gone
D. goes
城市生态系统是一个以______为中心的生态系统．
