In a relational database, two tables, 'Orders' and 'Customers', are related to each other. The 'Customers' table has a 'CustomerID' which uniquely identifies each customer. When this 'CustomerID' is used in the 'Orders' table to link each order to the specific customer that placed it, which kind of key does the 'CustomerID' represent in the 'Orders' table?
Index
Primary key
Unique key
Foreign key