Dry regions in the southwestern

Dry regions in the southwestern United States have become increasingly inviting playgrounds for the growing number of recreation seekers who own vehicles such as motorcycles or powered trail bikes and indulge(being crazy) in hill-climbing contests or in carving new trails in the desert. But recent scientific studies show that these off-road vehicles can cause damage to desert landscapes that has long-range effects on the area’s water-conserving characteristics and on the entire ecology (the scientific study of the pattern of relations of plants, animals, and people to each other and to their surroundings), both plant and animal. Research by scientists in the western Mojave Desert in California revealed that the compaction of the sandy dry soil resulting from the passage of just one motorcycle markedly reduced the infiltration(渗透)ability of the soil and created a stream of rain runoff water that eroded the hillside surface. In addition, the researchers discovered that the soil compaction caused by the off-road vehicles often killed native plant species and resulted in the invasion of different plant species within a few years. The native perennial (of a plant that lives for more than two years) species required many more years before they showed signs of returning . the scientists calculated that roughly a century would be required for the infiltration capacity of the Mojave soil to be restored after being compacted by vehicles.
What is the main topic of the passage?
A、Problems caused by recreational vehicle.
B、Types of off-road vehicles.
C、Plants of the southwestern desert.
D、The increasing number of recreation seekers.
【正确答案】：A