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...Had I known ... I would have....是什么意思Had I known ... I would have....是句子的格式比如Had I known you were coming, I would have waited for youHad I known it was going to rain, I would have brought an umbrella