You are managing a university database system and you need to handle optional information in the instructor table. The phone$\_$number field should allow for the possibility that an instructor's phone number is not provided.

Which SQL statement correctly alters the instructor table to ensure that phone$\_$number can handle NULL values, assuming the column does not already exist?

Group of answer choices

ALTER TABLE instructor ADD phone$\_$number VARCHAR$(15)$;

ALTER TABLE instructor ADD phone$\_$number VARCHAR$(15)$ NOT NULL;

ALTER TABLE instructor ADD phone$\_$number VARCHAR$(15)$ DEFAULT NULL;

ALTER TABLE instructor ADD phone$\_$number VARCHAR$(15)$ CHECK $($phone$\_$number IS NOT NULL$)$;