请阅读Passage 2，完成第小题。
Passage 2
Among China's greatest art treasures are the Buddhist caves near Dunhuang. Their ancient frescoes and sculptures have survived wars, environmental damage, antiquities hunters, and the chaotic Cultural Revolution.
Today domestic tourism is the biggest threat: the UNESCO World Heritage site has an optimal capacity of 3,000 per day, but peak times can see twice that many visitors.
The Mogao Grottoes are especially vulnerable to mass tourism. Their ecosystems are fragile. A buildup of humidity and carbon dioxide from visitors' breath can lead to flaking and discoloration of wall paintings.
To preserve the caves, the Dunhuang Academy is pioneering a project to digitize the site.
Recently, the Arthur M. Sackler Gallery in Washington, D.C., offered a tantalizing glimpse at the undertaking. Donning 3-D glasses, visitors were transported into a breathtaking "virtual" Dunhuang grotto, known as Cave 220. The 3-D, interactive experience is flooded with vivid color, close-up details, moving images of flying bodhisattvas, even sound, "Dunhuang ranks as the single most important repository of early Chinese art. Here the great cultures of the World--Greek and Roman,Persian and Middle Eastern, Indian and Chinese--constantly interacted for over a millennium,"said Mimi Gates, who formed the Dunhuang Foundation. "High-resolution digitization will provide a lasting record of this artistic treasure for all mankind and can make it accessible beyond China."
A dozen years ago, the Dunhuang Academy began cooperating with foregoing institutions to conserve the treasures. Among the projects, one used a camera to create a digital archive of the caves. The results will be used in the academy which planned ＄40 million state-of-the-art visitor center which will present virtual tour of the caves to save the real site wear and tear. The scope of the project is daunting. It requires 20 minutes or so to record a 9-square-meter
A.Antiquities hunters
B.Environmental damage
C.The optional number of tourists
D.Visitors exceeding the optimal number