Consider the following university database schema:

Student (SID, Name, Age, Major, GPA)

Course (CID, Title, Department, Credits)

Enrollment (SID, CID, Semester, Grade)

Professor (PID, Name, Department, Salary)

Teaches (PID, CID, Semester)

Which of the following relational algebra expression(s) will Find students who have taken both Course C1 and Course C2? (Note: there can be more than expressions)