1. Extend the previous exercise by implementing an addLetterGrade method. This is similar to addGrade except that it accepts a letter grade as a string (instead of a gradepoint). Use the updated class to improve the GPA calculator by allowing the entry of letter grades.

2. Write a modified Button class that creates circular buttons. Call your class CButton and implement the exact same methods that are in the existing Button class. Your constructor should take the center of the button and its radius as normal parameters. Place your class in a module called cbutton.py. Test your class by modifying roller.py to use your buttons.