The library of congress in Washington, D.C. which 【C1】______ the largest collection of boo

ks 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 tt3at 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.

【C1】

A．consists of

B．belongs to

C．houses

D．composes