请教关于reason用法

Rachmaninoff

25. One reason why more young people lose their virginity during the summer than at other times of the year undoubtedly is because school vacations give adolescents more free time.
(A) One reason why more young people lose their virginity during the summer than at other times of the year undoubtedly is because
(D) One reason more young people lose their virginity during the summer than at other times of the year is undoubtedly that

我看见cambridge词典说The reason why...is because 是对的，只是不正规．是不是ETS否认这种用法？　所以答案A错？　谢谢指点　

summertao

the reason.......is because.....在GMAT改错题里是标准错误答案。

chengshien

the reason for
(no) reason to do sth
for reasons of
the reason （why …）is that…