题目内容：
请阅读Passage 2，完成第26～30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 fresco, and there are 492 caves with murals inside. But the Sackler exhibit proved how enthralling the single virtual cave CaB be.
Real caves provide no lightbulbs. Once they reach critical levels of moisture and temperature,they are shut to the public. Only a few dozen caves are accessible at any given time. But the Sackler's virtual tour was different. One of the most popular features was the "magnifying glass",which can zoom in on, say, a zither depicted in a mural. The instrument appears to pop out of the wall, enlarge, and then rotate in space. Visitors can also "flip" back and forth between the intricate Tang-dynasty mural and a later, cruder Sung-dynasty fresco.
To help Cave 220's Tang dancer paintings magically come to life, two Chinese performers were flown to the Applied Laboratory for Interactive Visualization and Embodiment (ALIVE) in a Hong Kong university. For three days the dancers were filmed performing intricate steps, fluid movements, and careful manipulation of long, sinuous ribbons. They appeared in the Sackler tour,dancing as if in midair, clad in brightly colored Tang costume. ALIVE's project manager said while he's become intimately familiar with the images Cave 220, he hasn't been there yet. "I can't wait to visit the real thing."
Which of the following factors mentioned in the article may cause a severe damage to the Buddhist caves today? A．Antiquities hunters.
B．Environmental damage.
C．The optional number of tourists.
D．Visitors exceeding the optimal number.
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