完整的计算机系统由（）两大部分组成。

A、应用软件和系统软件

B、随机存储器和只读存储器

C、硬件系统和软件系统

D、中央处理器和外部设备

A man aboard a vessel，signaling by raising and lowering his outstretched arms to each side，is indicating（）.

A.danger，stay away

B.all is clear，it is safe to pass

C.all is clear，it is safe to approach

D.a distress signal