It was said was how the Chinese first raised silk worms.A.that whatB.that whichC.that whoD.that that
参考答案：
D解析本句中的it为形式主语，真正的主语是后面的由that 引导的主语从句that that was how the Chinese first raised silk worms，在这个主语从句中，第一个that 只起引导词的作用，第二个that 是指示代词，作从句的主语。
