Until late April, northern China had experienced eight severe sandstorms this year alone, worsening the air quality in half the region's cities. This year's sandstorms are considered more severe than before. Not only are they increasing in frequency and having more serious effects on air quality, but they are also occurring earlier in the season and affecting a wider area. The timing of the first sandstorm moved up by a week this year, and the sandstorm-affected area has extended beyond 3 million square kilometers. The reason for the frequent dust storms is worsening desertification in China's northern areas, caused by a combination of warm, dry weather and the rapid spread of agriculture. The dust-affected area is likely to expand to the northeast in the near future as a large area of grassland in Inner Mongolia's northern plain turns rapidly to desert. Due to its lack of rainfall and overall surface dryness in the spring, Beijing experiences sandstorms every March and April. The most recent sandstorm hit the capital on April 17 and was the worst in five years. For about 40 days every year, the pollutants cannot be diffused, as the city's location makes it a natural container for accumulating sand and dust. Rapid urban development is affecting Beijing's air quality as well. To  _ the dust from hundreds of new construction sites, authorities have issued rules asking builders to cover all working areas, pack down loose dirt, and wash vehicle tires when leaving the sites. All of the following are the reasons for desertification in northern China EXCEPT   _
warm and dry weather the rapid increase of agriculture. the disappearance of grassland the frequent dust storms