答案为A
从前文We 【didn't know】 his telephone number 可见,这是表示与过去事实相反的虚拟语气,主句应该是【would+ have + 过去分词】
而B 这种形式是表示与现在或者将来的事实相反虚拟语气在主句中的动词形式
楼主可能以为We didn't know his telephone number这个是条件句了,不是的.这是陈述语气,陈述的是过去的事实,后面的 otherwise"否则“ 相当于一个虚拟条件句.等于：If I had known his telephone number.had+过去分词 才是 would + have +过去分词吧刷新再看我的解释后面的 otherwise"否则“ 相当于一个虚拟条件句。 等于： If I 【had known】 his telephone number.不客气