Consider the following three relations: PROFESSOR (name specialization, salary) STUDENT (name, address, study-type) COURSE (number, stud-name, prof-name, grade) Write the SQL queries corresponding to the following questions. a. Find the number of different students who are taking courses. b. Find the average salary of professors. c. Find the number of courses tutored by each professor. d. Show the names and salaries of professors who teach student "John McDonald" in descending order of salary (use a subquery). descending order of salary (do not use a subquery) type = "individual". e. Show the names and salaries of professors who teach student "John McDonald" in f. Show the names and salaries of professors who have never taught a student with study- g. Show the names and salaries of professors who teach more than 20 different students