The National Gallery of Art in Washington, D.C., is one of the world's greatest art museums. Millions of people have entered its doors to see paintings by the world's fine artists. But if these priceless masterpieces are to be preserved, the Gallery must protect them carefully. The Gallery's 135-man guard force has successfully prevented them from being stolen, but protecting the paintings from nature is a greater problem. In past times, the owners of paintings did not protect them from damaging changes in humidity and temperature. As a result, the life of these paintings were shortened. In the National Gallery, however, humidity and temperature are carefully controlled. The building is air-conditioned in summer and heated in winter. The air-conditioning and heating system are so important to the life of the painting that the Gallery has two of each system. If one should fail, the extra one can take over. Light is another enemy of paintings. Ultraviolet rays in light cause paintings to fade . Long ago, paintings often hung in dark churches and palaces. A coat of varnish was a protection from the weak light. But when museums took over the care of many paintings, they were often hung in brighter light than before. Soon they were in danger of fading. The damaging effects of light were increased when the museums removed the varnish coating, yellowed with age. To protect its paintings, the National Gallery put a special kind of glass in its skylights. This glass allows visible lights to enter the building but it keeps out harmful ultraviolet rays. The Gallery has also developed new and better varnishes which help to keep paintings from fading. Thanks to these new _ , many of the world's greatest paintings are being well protected for future generations to enjoy. The text mainly tells us about  _  .
the guard force in the National Gallery protecting great paintings from nature priceless paintings of past times the air-conditioning and heating systems in the National Gallery
Question: The National Gallery of Art in Washington, D.C., is one of the world's greatest art museums. Millions of people have entered its doors to see paintings by the world's fine artists. But if these priceless masterpieces are to be preserved, the Gallery must protect them carefully. The Gallery's 135-man guard force has successfully prevented them from being stolen, but protecting the paintings from nature is a greater problem. In past times, the owners of paintings did not protect them from damaging changes in humidity and temperature. As a result, the life of these paintings were shortened. In the National Gallery, however, humidity and temperature are carefully controlled. The building is air-conditioned in summer and heated in winter. The air-conditioning and heating system are so important to the life of the painting that the Gallery has two of each system. If one should fail, the extra one can take over. Light is another enemy of paintings. Ultraviolet rays in light cause paintings to fade . Long ago, paintings often hung in dark churches and palaces. A coat of varnish was a protection from the weak light. But when museums took over the care of many paintings, they were often hung in brighter light than before. Soon they were in danger of fading. The damaging effects of light were increased when the museums removed the varnish coating, yellowed with age. To protect its paintings, the National Gallery put a special kind of glass in its skylights. This glass allows visible lights to enter the building but it keeps out harmful ultraviolet rays. The Gallery has also developed new and better varnishes which help to keep paintings from fading. Thanks to these new _ , many of the world's greatest paintings are being well protected for future generations to enjoy. The text mainly tells us about  _  .
A. the guard force in the National Gallery
B. protecting great paintings from nature
C. priceless paintings of past times
D. the air-conditioning and heating systems in the National Gallery
Answer: