Plastics are materials which are softened by heat and set into lasting form when shaped in a mold. Some are natural; some are semi-synthetic(半合成的) , the result of chemical action on natural substance; some are synthetic, built up from the constituents of oil or coal. All are based on the chemistry of carbon, with its capacity for forming chains. The molecules that compose them (monomers) link together in the setting or curing(硫化) process to form chains (polymers) , which give plastics their flexible strength. Some plastics retain their ability to be softened and reshaped; like wax, they are thermoplastic. Others set permanently in the shapes they are given by heat and pressure; like eggs, they are thermosetting.

　　From industrial beginnings in the nineteenth century, plastics have struggled through a hundred and twenty years of glory, failure, disrepute and suspicion on the slow road to public acceptance. Now, at last, one can positively say that plastics are appreciated and enjoyed for what they are; that they make modern life richer, more comfortable and convenient, and also more fun. Plastics are warm materials, sympathetic to the human touch, and their transformation into things that come into contact with human beings is entirely appropriate.

　　The fact that there are plastic antiques comes as a shock to most people. How can a material that seems so essentially twentieth century, and one that is so much associated with cheap, disposable products, has a history at all? It is a young technology, and a great part of the fun of collecting plastics is that beautiful pieces of historical interest can still be found very cheaply.

　　21. The word "sympathetic" in Paragraph 2 most probably means_______.

　　A. harmful B. agreeable

　　C. pitiful D. sorry

　　22. It can be concluded from this passage that_______.

　　A. plastics are synthetic materials

　　B. plastics won public acceptance 120 years ago

　　C. plastics are very harmful in modern life

　　D. plastics are cheap as antiques

　　23. Which of the following is essential to create any type of plastics? A. Carbon. B. Eggs.

　　C. Oil. D. Coal.

　　24. Plastics that harden into permanent shapes are called_______.

　　A. chained B. thermoplastic

　　C. synthetic D. thermosetting

　　25. Which of the following is the best title for the passage?

　　A. The Importance of Plastics in the Nineteenth Century.

　　B. Why People Are Suspicious of Plastics.

　　C. The Development of Plastics as a Modern Material.

　　D. How Plastics Are Manufactured.

　　21. B 22. D 23. A 24. D 25. C

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