HIs lecture was difficult and I didn't know -------he said meant in his lecture.A.that B.what C.tHIs lecture was difficult and I didn't know -------he said meant in his lecture.A.that B.what C.that that D.what waht 请问为什么选D

选C
首先你可以理解第一个that是从句的连接词。
第二个that，判断有点麻烦。先把后半句的语序颠倒回正常语序为：___he said in his lecture meant.然后that 作为代词出现代表他说的东西，再接上一个定语从句。所以整个句子应该是以下：
His lecture was diificult and I didn't know(主句) that(从句连词) that（代词做从句的主语（that / which）（定语从句连词，因为不做成分所以被省略）he said(定语从句主干)meant(从句谓语动词) in his lecture（定语从句中被后置的状语）
我个人是这么理解的，如果觉得有道理，望采纳~~~~~~

D

的确选D,先看看后面那个what--what he said,这的what指代的是said后面的内容,翻译出即使“他所说的”
前面那个what--what （what he said做这从句中的主语）meant in his lecture,这个what指代的是meant后面的内容,翻译出来是“（他所说的）在他的演讲中意味这的什么”
这个句子的意思是：他的演讲很难,我不知道他所说的在他的演讲中意味的什么.（即指我完全不懂他在说什么）

应该是填that what