You can 
put this solution on YOUR website! .
The words "select and rank" mean "to produce an ordered list".
In simple terms, the order in the list is important (does matter).
Therefore, there are  12*11*10*9 different lists/ways Kristen can do it:
    - any of 12 at the first position;
    - any of 11 remaining at the second position,
         . . .    and so on   . . . 
(standard mantra).