阅读理解 It may sound strange, but cutting down a real tree for Christmas is actually greener than going with the artificial kind, one scientist says. 'It is a little confusing to people,' said Clint Springer, a biologist at Saint Joseph's University in Philadelphia. Because of concerns over deforestation (砍伐森林) around the world, many people naturally worry that buying a real tree might contribute to that problem. But most Christmas trees for sale these days are grown not in the forest but on tree farms for the purpose of being cut. Moreover, from the viewpoint of greenhouse gases, real trees are 'the obvious choice'. Live trees actively release oxygen as they grow, and meanwhile remove carbon dioxide from the atmosphere. After they have been cut and Christmas is over, they're usually cut into smaller pieces for mulch (覆 盖物). As mulch, the bits of trees very slowly release carbon dioxide back into the atmosphere. So in the end, a real Christmas tree is carbon neutral , putting the same amount of carbon dioxide back into the air as it took out (although much more slowly). The tree farms that grow the trees also replant after the trees are cut. Artificial trees, on the other hand, don't come out even in the carbon balance. Petroleum (石油) is used to make the plastics in artificial trees and lots of carbon dioxide-creating energy is required to make and transport them. Because these trees just end up in the places where waste is buried under the ground after use, 'those greenhouse gases are lost forever,' Springer said. 1. The author presents the text mainly by . A. giving descriptions B. making comparisons C. listing figures D. analyzing different opinions 2. Real trees for Christmas are better than artificial trees probably because . A. real trees cost buyers less money B. real trees are more beautiful C. real trees don't cause any pollution D. real trees can be used for a longer time 3. The underlined part 'carbon neutral' in the third paragraph can be best replaced by ' '. A. not releasing more carbon B. absorbing carbon dioxide C. causing much waste D. releasing carbon dioxide 4. The purpose of the text is to tell us . A. the reasons for using real trees for Christmas B. the environmental protection tips for Christmas C. how to choose artificial Christmas trees D. how to spend a much greener Christmas