$1.$ Write a relational algebra expression that returns the number of students for each course $($use course$\_$id$)$ per year.

$2.$ Write a relational algebra expression that returns the title of Computer Science $($CS$)$ courses and their corresponding room number where these courses are taught in classrooms.

$3.$ Write a relational algebra expression that returns the department names and their corresponding budgets where the total number of students is larger than $200$ for the departments.

$4.$ Write a relational algebra expression that returns the student ID and name of the student$($s$)$ whose total credit is higher than the average total credits of all students.

$5.$ Write a relational algebra expression that returns the course ID and title of the courses that are taught by instructors with the highest salary. Hint: it may be useful to create variables to reduce the complexity of the final relational algebra expression