Devon Degosta is updating the employee database with information on a new employee. She enters the employee information into the employee table and then tries to add a department ID for a department that does not exist in the Departments table. The system does not allow her to add the employee record until she creates a Department ID first in the Department table so the employee can then be assigned to it$.$ This is an example of the system enforcing a

A$.$ referential integrity constraint

B$.$ scalability constraint

C$.$ data model

D$.$ business rule

E$.$ database constraint