Students(sid: integer, sname: string, major: string) Courses(cid: integer, cname: string) Enrollment(sid: integer, cid: integer, grade: real) Write a tuple relational calculus expression that satisfies this query? Find the distinct names of all students who major in either "Math" or "Economics" who score less than 60% in either course 91.573 or in course 14.601 (using cid).

What is the primary difference between the header section of a document and the body? a. the body is displayed on the webpage and the header is not. b. the header is displayed on the webpage and the body is not. c. the tag for the body is self-closing, but the tags for the headers must be closed. d. the tag for the header is self closing, but the tag for the body must be closed.