Part $6$ Invalid Input $(15$ points$)$

You notice that users are entering upper case letters, complete responses, and other hard to deal with input. To help them out, you will extend your program so that if the user enters something other than $\text{'}a\text{'},\text{'}b\text{'},$ or $\text{'}c\text{'}$ the program will immediately respond with

Invalid imput I Enter $a,b,$ or $c$ next t$$me$.$

Your program will go on to mark them incorrect.