单项选择题

Sandstorms hit northwestern China’s Gansu Province and North China’s Inner Mongolia Autonomous Region (内蒙古自治区) on the first day of the new century. According to Sun Landong, a meteorologist with the Lanzhou meteorology Observatory, visibility (能见度) was less than 100 meters in Minqin, a Gansu county near Inner Mongolia, because of sand. When visibility drops to less than 1 kilometer, it is called a sandstorm. The sandstorm in Minqin blew up dust in neighboring towns, such as Baiyu, Wuwei, Jichang and Lanzhou, the capital city of Gansu Province. Dust-laden winds also swept into Beijing. Brought by northwest lies blowing at more than 20 meters per second, dust first reached the capital’s the temperature down to as low as 8 below zero and many flights put off their planned time from the city’s airport.
It is quite rare for sandstorms, which are frequent between March and June, to happen in winter, when the earth is frozen. The sandstorms in Gansu and Inner Mongolia do not necessarily mean there will be more of them in the spring than last year, when they were serious, but Beijing is taking it very serious. It is reported that Beijing will spend 6 billion Yuan (US $ 725 million)in preventing sandstorms in the capital and has already set up a special team to make sure what causes them.

Suppose the dust was 300 kilometers away from Beijing, it would reach Beijing about（）hours later if it travels at a speed of 20 meters per second

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