Which of the following correctly states the referential integrity rule?

The primary key of a table must have data values $($cannot be null$).$

The foreign key of a table must have a data value $($cannot be null$).$

The data value for a primary key must either be null or match one of the data values that already exists in the corresponding table.

The data value for a foreign key must either be null or match one of the data values that already exists in the corresponding table.