Consider a university enrolments system with the following requirements. Students enrol in courses that are offered in particular semesters. Students receive grades (pass, fail, etc.) for courses in particular semesters. Courses may have prerequisites (other courses) and must have credit point values. For a student to enrol in a course, s/he must have passed all prerequisite courses. Course offerings are broken down into multiple sessions (lectures, tutorials and labs). Sessions in a course offering for a particular semester have an allocated room and timeslot. If a student enrols in a course, s/he must also enrol in some sessions of that course. (a) Draw a UML class diagram for an object-oriented system to implement the above requirements and provide CRC cards for your main classes. Clearly explain how your design distinguishes between a course and an instance of a course at a particular semester. (b) Define a use case for a student enrolling in a course that has a prerequisite that s/he has passed, and then define a walkthrough demonstrating how this would be handled in your system.