Most of the sandstorms that had swept China last year came from foreign land,a Chinese official in charge of desertification  control said on Monday. And the invasions could partly explain the frequent sandstorms in the country in recent years despite  its  achievements  in desertification control. Since the start of last spring,the north and northwestern Chinese regions had been hit by 17 sandstorms,of which,a dozen came from foreign land. Situated in the centralAsia sandstorm region,one of the world's four largest sandstorm sources,China also suffers from sandstorms from outside the country while being blamed as a sand source to northeast Asia. The other three major sources are in Africa,North America and Australia. The land suffering from desertification has been _ by 7,585 square kilometres annually in China,and the area of sandy land has also been falling by 1,284 square kilometres a year. The shrinkage forms a clear contrast to the fact that the land suffering from desertification and sandy feature was added by 10,400 square kilometres and 3,436 square kilometres late last century,respectively. Currently,the desertification land in China makes up 2.64 million square kilometres,accounting for 27.46 percent of the nation's land,and its sandy land totals 1.74 million square kilometres,accounting for 18.1 percent of the country's total. Which of the following is believed to be the sand source to northeast Asia?
Mongolia. Africa. China. Australia.