根据句意及首字母提示完成单词。

1. I thought hard and tried to get out of the e_____situation.

2. She didn't r_____her mistake until we told her.

3. If the bell r_____, we'll have to leave at once.

4. E_____is here, then let's start playing.

5. I've n_____been to the USA before.

IBM PC微型计算机中，有符号数是用______表示的。 ( )

A．原码

B．补码

C．反码

D．BCD码