In a relational database designed for a healthcare clinic, there are two tables: 'Patients' and 'Appointments'. The 'Patients' table has a 'PatientID' field, which uniquely identifies each patient. How should this field be properly used in the 'Appointments' table to maintain a relationship between the two tables?
Copy all patient information into the 'Appointments' table for each appointment
Generate a random unique identifier for each appointment separately from 'PatientID'
Include 'PatientID' as a 'Foreign Key' in the 'Appointments' table
Create a new 'AppointmentID' field in the 'Patients' table