When ____ the education systems of China and Britain,the professor gave a detailed comparison betwee

题目

When ____ the education systems of China and Britain,the professor gave a detailed comparison between the two.
选项:
a、being asked to compare
b、asked him to compare
c、asking him to compare
d、asked to compare

答案

a、being asked to compare很显然,是被问.被动句.the professor 是主语BC显然不对.如果用D：When ____ the education systems of China and Britain,这个从句,应该用完整的形式.he was asked to compare...