protected function correct_choices(array $right) {
        // Return appropriate string for single/multiple correct answer(s).
        if (count($right) == 1) {
                return get_string('correctansweris', 'qtype_multichoice',
                        implode(', ', $right));
        } else if (count($right) > 1) {
                return get_string('correctanswersare', 'qtype_multichoice',
                        implode(', ', $right));
        } else {
                return "";
        }
    }