Refer to the exhibit.

A data architect builds a simple data model to show the relationship between students and exams. The data is loaded. Every StudentID in the Exams table should be found in the Students table. Some students have NOT taken an exam.

The data architect selects the field "StudentID" from the Students table and sees the following:

A data architect needs to fix this anomaly.

What should the data architect do to ensure data integrity?