Sulfur dioxide is the main cause of acid rain. In order to improve the situation, a certain area is suffering from acid rain. In order to improve the situation, the region reduced a total of 118,000 tons of sulfur dioxide from January to June, a year-on-year decrease of 9.1%. The content decreased, but the frequency of acid rain increased by 7.1%. Which of the following can best explain this phenomenon:. ['Part of the sulfur dioxide in the air in the area drifted from the surrounding area.', 'Although the emission of sulfur dioxide is controlled, its effect will take some time to achieve.', 'A large increase in motor vehicles has exacerbated the emission of nitrogen oxides, which are also an important cause of acid rain.', 'Although the total amount of sulfur dioxide emissions has decreased, the proportion of sulfur dioxide in pollutants has not changed.']