Mr. Green is said an experiment to prove the new method of solving the problem when young.
A. to do
B. to have done
C. to be doing
D. to have been doing
网友回答
A. to do
(答案→)B. to have done
C. to be doing
D. to have been doing
解析：由when young可知此处应该用不定式的完成式，表示此动作发生在谓语动作之前。