David’s effort was ______, because he won the first prize in the English speech contest, which made his classmates surprised.                                          

A．paid off

B．paid back

C．paid for

D．paid up

程序测试分为静态分析和动态测试。其中______是指不执行程序，而只是对程序文本进行检查，通过阅读和讨论，分析和发现程序中的错误。