用户使用操作系统通常有三种方式：终端命令、系统调用命令和______。

A．计算机指令

B．宏命令

C．作业控制语盲

D．汇编语言

If you must land on a beach with an oar-propelled lifeboat through a heavy surf，the recommended method is to（）.

A.keep the bow directly in toward the beach，and tow the sea anchor off the stern

B.ride in on the back of a large breaker

C.keep the bow into the seas with the sea anchor out over the bow，and row to meet the breaking waves

D.head directly into the beach by staying between the crests of the waves