．Walking together, my father and I used to have a lot of conversations I __________learned lessons from his experience.

A．that

B．where

C．when

D．through which

铸铁管带压开孔操作时，要求把钻孔工具稳固地置于铸铁管上，在钻孔机座和管壁之间垫入带孔的橡胶板以保持（）。

A、稳定

B、安全

C、密封

D、防腐