请教关于reason用法

thornbirdcfa

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　

[此贴子已经被作者于2004-1-6 4:54:14编辑过]

dansy

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

dreadpower

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

zhang711228

redundancy