问题补充:
It was said was how the Chinese first raised silk worms.
A. that what
B. that which
C. that who
D. that that
答案:
A. that what
B. that which
C. that who
(答案→)D. that that
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。