public function is_open( $user_id = null ) {

		$student = llms_get_student( $user_id );
		if ( ! $student ) {
			$quiz_open = false;
		} else {

			$remaining = $student->quizzes()->get_attempts_remaining_for_quiz( $this->get( 'id' ) );

			// string for "unlimited" or number of attempts.
			$quiz_open = ! is_numeric( $remaining ) || $remaining > 0;

			// Check for a passed attempt and disable the quiz.
			if ( $quiz_open && llms_parse_bool( $this->get( 'disable_retake' ) ) ) {
				$passed_attempts = $student->quizzes()->get_attempts_by_quiz(
					$this->get( 'id' ),
					array(
						'status' => array( 'pass' ),
					)
				);

				if ( count( $passed_attempts ) ) {
					$quiz_open = false;
				}
			}
		}

		/**
		 * Filters whether the quiz is open to a student or not.
		 *
		 * @param boolean            $quiz_open Whether the quiz is open.
		 * @param int|null           $user_id   WP User ID, can be `null`.
		 * @param int                $quiz_id   The Quiz id.
		 * @param LLMS_Quiz          $quiz      The LLMS_Quiz instance.
		 * @param LLMS_Student|false $student   LLMS_Student instance or false if user not found.
		 */
		return apply_filters( 'llms_quiz_is_open', $quiz_open, $user_id, $this->get( 'id' ), $this, $student );
	}