A granite sales company keeps information on employees and the departments that the employees work in$.$ For each department, the department number $($the identifier$),$ the department name, internal mailbox number, and office phone extension are kept. A department can have many assigned employees, and each employee is assigned to only one department. Data is also needed for newly joined employees $-$ these employees do not have a department. A department needs to have at least one employee. All employees

are assigned an employee number $($the identifier$).$ This is kept along with the employee's name and address.

a$.$ Draw a project$-$level ER diagram for the above situation.

b$.$ Which attribute will act as a foreign key in this relationship?