They have started an investigation, ________ are being kept secret. [ ]

A. details of which

B. of which the details

C. and the detail

D. the details of it

市检修公司检修试验工区主要负责城（郊）区（）变电站设备专业化检修、试验校验、异常（故障）诊断分析、应急抢修等工作。

A、10千伏；

B、35千伏；

C、110千伏；

D、220千伏