A tuple of EMPLOYEE_EXTENSION represents the unique identifier of an employee, the employee's office number and the employee's phone extension. EMPLOYEE_EXTENSION EMPLOYEE ID E1 E1 E2 E3 E5 OFFICE R101 R102 R10 R35 R35 EXTENSION 811 813 111 123 123 a) Identify the relation defined by the following relational algebra expression. PROJECT EMPLOYEE_EXTENSION ON EXTENSION b) What is the primary key of the relation EMPLOYEE_EXTENSION