Passage Two Questions 26 to 30 are based on the following passage.
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 readily. 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 alone, 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 becoming 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 0selected 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.
26. Why is the close planting of trees of the same species avoided in modern orchards and forests? 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.
27. The relative stability of the human population has been upset by____. A) the proportion of the reproductive age group B) the influenza epidemic C) advances in medical science D) the Black Death in Europe
28. The number of the bellbird in New Zealand got controlled by____. A) the decrease in distribution B) human predation C) deliberate introduction of its enemy animal D) spread of certain kind of disease
29. Attempts to introduce myxomatosis into New Zealand to control rabbit population have failed, mainly because____. A) increasing resistance to the disease is becoming apparent B) there are no species of insects transmitting the disease in this country C) flea and mosquito without the disease are absent in this country D) the myxomatosis virus has been introduced into Australia
30. Which of the following can be inferred from the passage? A) Advances in medical science made disease unable to regulate human populatio n growth any longer in many parts of the world. B) Medical cures have prolonged life but upset the reproductive age group by increasing many populations. C) In Australia, the use of the myxomatosis virus hardly controlled the rabbit population with increasing resistance to the disease. D) It is discovered all strains of virus can parasitize and kill the bacteria without harming the organism that the bacteria has infected.