正确答案：选C.couldn't have
即：I did not know you were good friends.
You couldn't have.I have known her since she moved here.You were abroad then.
翻译：我原来不知道你们是好朋友.
你不可能知道.自从她搬到这儿我就认识她了,那时你出国了.
解析：首先纠正一下你的句子中有拼写错误的单词,dood应该是good,science应该是since 自从.couldn't have done 是指对过去做过的事情的否定推测,表示不可能...的意思；根据句意应该选C项,指原来不可能知道这件事.而may have done 是指对过去做的事情的肯定推测,原来有可能...；needn't have done 是指过去本不应该做的事情却做了；must have done 是指对过去做的事情的肯定推测,而且可能性要比may have done要大.
有不明白的请继续追问,