Let \( S \) be the specification: "Instructors teach courses. Students register for courses. Courses are allocated classrooms. Instructors guide students." Which one of the following ER diagrams CORRECTLY represents \( S \)?
\includegraphics[width=6.5cm]{21a.png}
\includegraphics[width=6.5cm]{21b.png}
\includegraphics[width=6.5cm]{21c.png}
\includegraphics[width=6.5cm]{21d.png}