If needed, please ask clarification questions.

Please do following:

1. Fork repository
2. Change project code to make MultipleUsersTest pass
3. Change project code to make AllQuestionsValidTest pass
4. Refactor project code and tests to your liking and best effort . Focus on QuizController and QuizService.
5. Write tests for QuizService and QuizController
6. Prepare and submit pull request

Try to do every step as a separate commit.