I didn't know his telephone numberA.Had I known; would ring him upB.Should I know ;would have ring him upC.If I knew ;would ring him upD.Had I know; would have rung him up请问这道题的选项为什么是D?

I didn't know his telephone numberA.Had I known; would ring him upB.Should I know ;would have ring him upC.If I knew ;would ring him upD.Had I know; would have rung him up请问这道题的选项为什么是D?
I didn't know his telephone number
A.Had I known; would ring him up
B.Should I know ;would have ring him up
C.If I knew ;would ring him up
D.Had I know; would have rung him up
请问这道题的选项为什么是D?

I didn't know his telephone numberA.Had I known; would ring him upB.Should I know ;would have ring him upC.If I knew ;would ring him upD.Had I know; would have rung him up请问这道题的选项为什么是D?
答案是 Had I known,I would have rung him up
原句 D 好像写错了,
如果 D 的形式是 Had I known,I would have rung him up
选 D .
Had I known = If I had known
是倒装,表示虚拟语气
I would have rung him up
过去将来完成时,配合主句的时态
提示：Had I known ...I would have ...
是固定搭配.