________ the arrival of the police, I would have been robbed by two masked men.

A．Had it not been for

B．If it were not for

C．Had it been not for

D．If it would not be for

为保证网管系统的稳定运行，应定期检查各服务器磁盘空间利用率，常用查看UNIX系统文件系统利用率的命令是（）。