The job required        did it        careful and brave enough.

A．who; is

B．whom; was

C．whomever; were

D．whoever; be

零售管理系统中前台收银机经常死机，可能的故障原因是：顾客显示屏问题引起、扫描枪问题引起、（）。

A、收银电脑设置了频幕保护

B、网络版网线已经断开

C、收银机主板故障引起

D、数据传输故障引起