Study the class below:

class Validate$\{$

private static $usernameExp $="/^[[$:lower:$]]\{4\}$$$/"$;

private static $passwordExp $="/^[[$:digit:$]]\{6,8\}$$$/"$;

public static function checkUsername$($$un$)\{$

return preg$\_$match$($self::$usernameExp,$un$)$;

$\}$

public static function checkPassword$($$pd$)\{$

return preg$\_$match$($self::$passwordExp,$pd$)$;

$\}$

$\}$

Which of the following is correct with respect to this class?