public function get_correct_response() {
        $response = $this->question->get_correct_response();
        if (is_null($response)) {
            return null;
        }
        $imvars = $this->behaviour->get_correct_response();
        foreach ($imvars as $name => $value) {
            $response['-' . $name] = $value;
        }
        return $response;
    }