Task: Question Answering
Input (English): Passage: Although the Silk Road from China to Europe and the Western World was initially formulated during the reign of Emperor Wu (141–87 BC) during the Han, it was reopened by the Tang in 639 when Hou Junji (d. 643) conquered the West, and remained open for almost four decades. It was closed after the Tibetans captured it in 678, but in 699, during Empress Wu's period, the Silk Road reopened when the Tang reconquered the Four Garrisons of Anxi originally installed in 640, once again connecting China directly to the West for land-based trade. The Tang captured the vital route through the Gilgit Valley from Tibet in 722, lost it to the Tibetans in 737, and regained it under the command of the Goguryeo-Korean General Gao Xianzhi. When the An Lushan Rebellion ended in 763, the Tang Empire had once again lost control over its western lands, as the Tibetan Empire largely cut off China's direct access to the Silk Road. An internal rebellion in 848 ousted the Tibetan rulers, and Tang China regained its northwestern prefectures from Tibet in 851. These lands contained crucial grazing areas and pastures for raising horses that the Tang dynasty desperately needed.Despite the many expatriate European travelers coming into China to live and trade, many travelers, mainly religious monks and missionaries, recorded the strict border laws that the Chinese enforced. As the monk Xuanzang and many other monk travelers attested to, there were many Chinese government checkpoints along the Silk Road that examined travel permits into the Tang Empire. Furthermore, banditry was a problem along the checkpoints and oasis towns, as Xuanzang also recorded that his group of travelers were assaulted by bandits on multiple occasions.
The Silk Road also affected Tang dynasty art. Horses became a significant symbol of prosperity and power as well as an instrument of military and diplomatic policy. Horses were also revered as a relative of the dragon. 
Question: In what year was the Silk Road reopened?
Output (English): 639.