The correct answer is A. Disappearing forests and the overuse of farmland have caused the Gobi Desert to spread across China.
The Gobi Desert is a large desert region located between northern China and southern Mongolia. It can be considered one of the largest and most important deserts in the world.
Nowadays, the Gobi Desert is in a growing process, due to a combination of factors, among them the increase of the population and its corresponding greater economic activity: the overgrazing and a prolonged drought that have contributed to the expansion of the surface of the Gobi Desert at a speed of three kilometers a year. To try to stop the advance of the desert, the Chinese government is implementing a Green Wall, a large afforestation project that is expected to be completed by the year 2074, when it would reach a length of 4500 kilometers.