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  3. Disease can be one of the most powerful factors in checking population growth. In crowded conditions where many individuals of a

Disease can be one of the most powerful factors in checking population growth. In crowded conditions where many individuals of a

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问题      Disease can be one of the most powerful factors in checking population growth. In crowded conditions where many individuals of a species are living close together, the spread of pathogens(病原体)from one individual to another, occurs read fly. History shows many instances where human populations, crowded together in cities, have been almost wiped out by the rapid spread of disease. Modern orchard and forestry practice recognizes this fact and the close planting of trees of the same species is avoided. Mixed forests and orchards are the recognized procedure these days.
    Apart from regulating population numbers in other species, disease has probably been the greatest factor in controlling the growth of the human population. In the mid-14th century, the Black Death wiped out 25 million people in Europe a lone, while as recently as 1918 over 21 million people died in a single year as a result of the influenza epidemic. The relative stability of the human population has been upset by advances in medical science. No longer does disease regulate human population growth in many parts of the world. Medical cures have prolonged life and upset the age structure of many populations, increasing the proportion of individuals in the reproductive age group.
    The decrease in distribution and numbers of some species of native birds in New Zealand-the bellbird for example-has been attributed to disease rather than predation. Indeed, there are examples to show that disease has deliberately been used to control some animal populations. In Australia, for instance, the introduction of the myxomatosis(多发粘液瘤病) virus has drastically reduced the rabbit population in many areas, although increasing resistance to the disease is be coming apparent. Attempts to introduce myxomatosis into New Zealand as a means of rabbit population control have failed, mainly because the species of flea and mosquito that transmit the disease are absent in this country.
    Research is currently being carried out to discover whether selected strains of virus can be used to control pathogenic bacteria that have developed a resistance to drugs. It is hoped that the virus will parasite(寄生于)and kill the bacteria without harming the organism that the bacteria has infected.
选项 A、Because mixed forests and orchards are the recognized procedures these days.
B、Because trees of same species may cause the spread of disease.
C、Because modern orchards and forests are more specialized.
D、Because trees planted crowdedly do not have enough room to develop their roots.
答案B
解析 因果关系题。见文章第一段第二句话In crowded conditions where many individuals of a species are living close together，the spread of pathogens(病原体)from one individual to another，occurs readily．说明同一物种生存的距离太近会造成疾病传染，所以答案为B。
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