Прочитайте текст и выберите правильный ответ: The world of outdoor advertising: billboards, transport and ‘street furniture’ (things like bus shelters and public toilets )—is worth about $18 billion a year, just 6% of all the world’s spending on advertising. But it is one of the fastest-:growing segments, having doubled its market share in recent years. What does the number 18 refer to?