In recent years, the weather of floating dust, sand blowing and sandstorm affecting Beijing is increasing, but most of the sand sources are not from Beijing, and the sand sources are the most.

The research shows that there are three main paths for dust to affect Beijing. The first path is the sandland area in Shandake, Inner Mongolia-Heihe River Basin in Hebei-Beijing area; The second path is Zhu Rihe in Inner Mongolia-Yanghe River Basin in Hebei-Yongding River Basin in Beijing; The third path is the Sanggan River Basin in Hebei Province-Yongding River Basin in Beijing.

So choose: a.