Fifty years after Einstein’s death,a question is asked,“Will there ever be another Einstein?”

Fifty years after Einstein’s death,a question is asked,“Will there ever be another Einstein?”
( )l 3．Why hasn't there been another scientist as great as Einstein?
A．Because the study of physics is not enough．
B.Because it is hard to make important discoveries as Einstein did．
C．Because people don't work hard any more．
D．Because physics is getting more and more difficult．
请帮忙说明为何不是D?