The chairman thought        necessary to invite Professor Smith to speak at the meeting.      [ ]

A. that              

B. it

C. this              

D. him

防止运行中变压器油劣化措施有（）

A、变压器加装虹吸过滤器

B、运行中变压器油添加抗氧化剂

C、变压器密封充氮

D、变压器采用油枕隔膜密封保护