请问下面那句英语用法是对的：
问题描述：
请问下面那句英语用法是对的：
1、He must either be very clever or foolish.
2、He either must be very clever or foolish.
3、He must be either very clever or foolish.
4、He not only must be a student but a boy as well.
5、He must be not only a student but a boy as well.
6、He must not only be a student but a boy as well.
答
3