2. 外语
  3. The library of congress in Washington, D.C., which【C1】______ the largest collection of books in the world, is fighting a battle

The library of congress in Washington, D.C., which【C1】______ the largest collection of books in the world, is fighting a battle

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问题      The library of congress in Washington, D.C., which【C1】______ the largest collection of books in the world, is fighting a battle against paper deterioration. The【C2】______ of the old books, often yellowed and torn,【C3】______ crumble when they are touched. The main culprit in the battle is the acidic paper that has【C4】______ making books since the nineteenth century.
     Air pollution and moisture【C5】______ the problem.【C6】______ , the books that are most【C7】______ destruction are not the oldest books. The paper in books produced【C8】______  the last century was made【C9】______ cotton and linen rags,【C10】______ are naturally low in acid. And the Gutenberg bible,【C11】______ five centuries ago, was made of thin calfskin, and is in【C12】______  good shape. But in the nineteenth century, with widespread literacy bringing a demand for a cheaper and more plentiful【C13】______ of paper, the industry began using  【C14】______  treated wood pulp for making paper.【C15】______ the chemical in these paper that is causing today’s problem. This problem of paper deterioration is one of【C16】______ concern. France, Canada, and Austria are all doing research into the new methods of deacidification. A new technology has been【C17】______ recently, in fact, that allows for mass deacidification of【C18】______ books at the same time. It costs less than microfilming and  【C19】______ preserves books in their original form.【C20】______ there will soon be treatment facilities all over the world to preserve and deacidify library book collections.
【C3】
选项 A、at a time
B、sometime
C、sometime
D、sometimes
答案D
解析 此句的意思是：一些书的纸张已经破旧发黄，碰到就会破损。at a time：一次；at times也可以表示有时；some time是指相当长的一段时间；some time指某个时间。因此正确答案是D。
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