Question 650801
AB, AC, AD, BA, BC, BD, CA, CB, CD, DA, DB, DC

There are 12.  That's because there are 4 ways to choose the first letter.
For each of those 4 ways to choose the first letter, there remain only 3
ways to choose the second letter (because you cannot choose the same letter
that you chose for the first letter).  That's 4×3 or 12 ways, and they are
listed above.

Edwin</pre>