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I didn't know his telephone number.________ it, I ________ then.
If I had known; would have rung him up
Should I know; would have rung him up
If I knew; would ring him up
Had I known; would ring him up
I didn’t know his telephone number._________ it, I _________ then.
Had I known, would ring him up
Should I know, would have rung him up
Had I known; would have rung him up