Consider the following (normalized) relational model (primary keys underlined; foreign keys in italics):

a. A department always has exactly one manager.

b. Every employee must always be supervised by exactly one other employee.

c. Every project is always assigned to exactly one department.

d. According to the model, an employee can work in another department than he/she manages.

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EMPLOYEE (SSN, ENAME, EADDRESS, SEX, DATE_OF_BIRTH, SUPERVISOR, DNR) SUPERVISOR: foreign key, refers to SSN in EMPLOYEE, NULL value allowed DNR: foreign key, refers to DNR in DEPARTMENT, NULL value not allowed DEPARTMENT (DNR, DNAME, DLOCATION, MGNR) MGNR: foreign key, refers to SSN in EMPLOYEE, NULL value not allowed PROJECT (PNR, PNAME, PDURATION, DNR) DNR: foreign key, refers to DNR in DEPARTMENT, NULL value not allowed WORKS ON (SSN, PNR, HOURS) SSN: foreign key, refers to SSN in EMPLOYEE, NULL value not allowed PNR: foreign key, refers to PNR in PROJECT, NULL value not allowed Which statement is not correct?