Do you know — In the 6th century.A.who is the abacus invented B.
问题描述：
Do you know — In the 6th century.A.who is the abacus invented B.
Do you know — In the 6th century.A.who is the abacus invented B.when the abacus is invented C.when was the abacus invented D.when the abacus was invented
答
选D～～～～～～～～～～～～～算盘是什么时候发明的．was不用改成is吗？既然回答是在过去的时间里发生的．问句只能体现在谓语动词里了．所以用WAS不是有do吗when 引导的从句与主语是可以分开来的． 作从句的主语为算盘的句子是个被动句，结构为：be+Ved的形式