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 Retrieve the names of professors who teach at least one course? (Note: there can be more than expressions)