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　　Although pathogenic organisms constantly alight on the skin, they find it a very unfavorable environment and, in the absence of injury, have great difficulty colonizing it. This “self-sterilizing” caPACity of the skin results from the tendency of all well-developed ecosystems toward homeostasis, or the maintenance of the status quo. Species that typically live in soil, water, and elsewhere rarely multiply on the skin. Undamaged skin is also unfavorable to most human pathogens. The skin is too acid and too arid for some species. The constant shedding of the surface skin layers further hinders the establishment of invaders. The most interesting defense mechanism, however, results from the metabolic activities of the resident flora (resident flora: 聚居植物群). Unsaturated fatty acids, an important component of the lipids in sebum collected from the skin surface, inhibit the growth of several bacterial and fungal cutaneous pathogens. These acids are a metabolic product of certain gram-positive members of the cutaneous community, which break down the more complex lipids in freshly secreted sebum.
　　17. The primary purpose of the passage is to
　　(A) offer an analysis of metabolic processes (B) detail the ways in which bacteria and fungi can be inhibited (C) describe mechanisms by which the skin protects itself against pathogens (D) analyze the methods whereby biological systems maintain the status quo (E) provide a specific example of the skin’s basic defenses against pathogens
　　18. The “resident flora” mentioned in line 16 refer to
　　(A) “Unsaturated fatty acids” (line 17) (B) “sebum collected from the skin surface” (lines 18-19) (C) “bacterial and fungal cutaneous pathogens” (lines 19-20) (D) “certain gram-positive members of the cutaneous community” (lines 21-22) (E) “more complex lipids” (lines 23)
　　19. Among the natural defenses of the skin against pathogenic organisms are all of the following EXCEPT the
　　(A) dryness of the skin (B) acidity of the skin (C) tendency of the pathogens toward homeostasis (D) shedding of surface layers of the skin (E) metabolic breakdown of lipids
　　20. The author presents her material in which of the following ways?
　　(A) Stating a problem and then supplying a solution (B) Presenting a phenomenon and then analyzing reason for it (C) Providing information and then drawing a conclusion from it (D) Making a general statement and then arguing by analogy (E) Making an inference and then developing it by illustration