Identify the best primary keys in these relations: 1. STUDENT(studentld, firstName, lastName, dateOfBirth, address) 2. STUDENT_COURSE(studentld, courseld, semester), assuming students cannot retake any course 3. STUDENT_COURSE(studentId, courseld, semester), assuming students can retake courses 4. LECTURER(lecturerld, firstName, lastName, dateOfBirth, address) 5. LECTURER_COURSE(lecturerld, courseld, semester), assuming each course is only taught by ONE lecturer, but courses can be taught by different lecturers in different semesters. 6. LECTURER_COURSE(lecturerld, courseld, semester), assuming there are more than one lecturer in some courses, and that the courses can be taught by different lecturers in different semesters.