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

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Question $5$

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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 Student$/$D values in STUDENT that do not appear in ENROL.