4. (15 points) Relational Algebra and Queries. Consider the following relational schema (keys are underlined): Students (sid: integer, sname: string, major string) Takes (sid: string, cid: string, grade: string) Courses (cid: string, title: string) (a) Write the following query in relational algebra: Find the names of the students who major in Math or CS, where `Math' and 'CS' are the values of the attribute major in the Students relation. (b) Write the following query in relational algebra: Find the names of the students who are enrolled in CSC411 but not in CSC414, where 'CSC411' and 'CSC414' are the values of the attribute title in the Courses relation. () Write the following query in relational algebra: Find the names of the students who are enrolled in both 'CSC411' and 'CSC414, where 'CSC411' and 'CSC414' are the values of the attribute title in the Courses relation