In many universities, graduate students are employed to teach introductory undergraduate courses,…

In many universities, graduate students are employed to teach introductory undergraduate courses, even though the full professors at these universities have more experience and could potentially teach these courses better. Which BEST explains why universities choose to hire graduate students instead of full professors to teach their introductory courses?
Graduate students gain a deeper understanding of the material by teaching it .
The opportunity cost of teaching an introductory course is higher for full professors than for graduate students.
The opportunity cost of teaching an introductory course is higher for graduate students than for full professors.
Introductory classes are the best place for graduate students to build their teaching skills.