Specify the following queries in SQL on the database schema of Figure $2.$

a$)[7$ marks$]$ Get the names of the courses that have two or more sections.

b$)[7$ marks$]$ Get the names of the instructors who are teaching at least two different courses in the same semester of the same year.

c$)[5$ marks$]$ Retrieve the names of students who took only one course in the Spring $2020$ semester.