Why is the following query NOT valid as a NATURAL JOIN?

SELECT customer.first$\_$name, customer.last$\_$name, employee.first$\_$name, employee.last$\_$name

FROM customer

NATURAL JOIN employee;

The tables do not have a foreign key relationship.

The syntax used for NATURAL JOIN is not correct.

The tables have a shared column, but their column names are not identical, so a JOIN... ON is needed.

These two tables do not share any identical columns$/$fields$.$