Which string can't be accessed? [closed]

We start from the string $ABCDEF$ and we can edit it with the following rules:

1.$ABCDEF \Rightarrow ADBECF$

2.$ABCDEF \Rightarrow DAEBFC$

Which string can't be accessed?

1. DBAFEC
2. FCBEDA
3. CABEFD
4. EFDCAB
5. FEDCBA

In these kind of problems we usually look for a common thing in every change of the the string, which I can't do here.

According to the book, the answer is

string $5$.

• For content that is not your own, please provide attribution. Using someone else's content without (at minimum) disclosing where it came from is plagiarism, and such posts are deleted. – Rubio Sep 25 '17 at 18:22
• Not to mention cross posting is generally discouraged without good reason. – Bobson Sep 25 '17 at 20:21