What does the following SQL query do$?$

SELECT employee$\_$id FROM employees UNION

SELECT employee$\_$id FROM former$\_$employees;

$$ Retrieves all employee IDs from both 'employees' and 'former$\_$employees' tables, removing duplicates.

$$ Retrieves all employee IDs from the 'employees' table.

$$ Retrieves employee IDs from both 'employees' and 'former$\_$employees' tables, removing duplicates.

$$ Retrieves employee IDs from both 'employees' and 'former$\_$employees' tables, including duplicates.