The man didn't know ________ the thief broke into the house and stole all his valuable things. [      ]

A. it was when that  

B. when it was that

C. that it was when  

D. was it when that

信号集中监测系统的操作人员需要具备一定的信号知识和计算机知识，了解主要信号设备的工作原理和（），了解信号设备日常巡检和维护流程。