Consider the relation schemas where we have made bold the primary keys:

STUDENT$($StudentID$,$ Name, DoB, Email$)$

ENROL$($StudentID$,$ CourseNo, Semester, Status, EnrolDate$)$

The foreign key StudentID in ENROL references StudentID in STUDENT.

Which of the following statements is incorrect?

$$ a$.$ The StudentID values in ENROL must be distinct.

$$ b$.$ StudentID must be a superkey in STUDENT.

$$ c$.$ There may exist StudentID values in STUDENT that do not appear in ENROL.