Replacing letters from input with a changing item from a list : In python, I'm trying to write a python script that replaces each letter with another letter from a 3 item long **list**. I am doing this for each letter of the alphabet, so use the `replace` method.   
For example:  
`list_a = "B,C,D" % `  
`$ what is your message? > aaaa`  
`$ This is your encrypted message: BCDB`  
Thanks a lot!!!!