—How did the accident happen?

—You know, it was difficult to see the road clearly because it       . [ ]

A. is raining        

B. rains  

C. was raining            

D. has rained

下列关于软件的描述中，错误的是( )。

A．可分为系统软件和应用软件

B．系统软件的核心是操作系统

C．共享软件的作者不保留版权

D．自由软件可自由复制和修改