While sales of other highly fuel-efficient automobiles are in decline, sales of the Hydro are rising. The Hydro' s manufacturers attribute its success to the Hydro' s price and very low fuel consumption. However, the Hydro is comparable in price and fuel efficiency to its competitors, so it is more likely that its success is due to the fact that people want to appear environmentally conscious to their neighbors. Which one of the following is an assumption required by the argument? ['Hydro buyers are more likely to have neighbors who also drive Hydros.', 'Hydro buyers have less interest in environmental causes than buyers of other highly fuel-efficient automobiles.', 'The Hydro is recognizable as environmentally friendly in a way that its competitors are not.', 'The Hydro is the most popular highly fuel-efficient automobile available.']