考研考试

【单选题】Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or contract over historical time intervals (tens to hundreds of years), species’’ ranges rarely expand thousands of miles or across physical barriers such as oceans or mountains.　　Habitat modification can create conditions favorable to the establishment of non-indigenous species. Soil disturbed in construction and agriculture is open for colonization by non-indigenous weeds, which in turn may provide habitats for the non-indigenous insects that evolved with them. Human-generated changes in fire frequency, grazing intensity, as well as soil stability and nutrient levels similarly facilitate the spread and establishment of non-indigenous plants. When human changes to natural environments span large geographical areas, they effectively create passages for species movement between previously isolated locales. The rapid spread of the Russian wheat aphid to fifteen states in just two years following its 1986 arrival has been attributed in part to the prevalence of alternative host plants that are available when wheat is not. Many of these are non- indigenous grasses recommended for planting on the forty million or more acres enrolled in the U.S.Department ofAgricultureConservation Reserve Program.　　
A、number of factors perplex quantitative evaluation of the relative importance of various entry pathways. Time lags often occur between establishment of non-indigenous species and their detection, and tracing the pathway for a long-established species is difficult.Experts estimate that non-indigenous weeds are usually detected only after having been in the country for thirty years or having spread to at least ten thousand acres. In addition, federal port inspection, although a major source of information on non-indigenous species pathways, especially for agriculture pests, provides data only when such species enter via closely-examined routes. Finally, some comparisons between pathways defy quantitative analysis-for example, which is more "important": the entry path of one very harmful species or one by which many but less harmful species enter the country Which of the following may best express the chief purpose of the last paragraph.’’
A、 To explain the difficulties in tracing the pathways for long-established species.
B． To describe the events leading to the detection of non-indigenous species.
C． To identify the problems in assessing the weight of entry tracks for foreign species.
D． To discuss the role of time lags and geographic expansion in species detection.

网考网参考答案：C
网考网解析：
末段首句点明了此段的主旨与目的：“有许多因素使得对非本土物种进入 (美国)的各种途径的相对重要性的定量评价复杂化了。”然后分析了这些因素。C项说法符合此意，其中problems相当于原句中的factors；assessing相当于 evaluation；weight相当于importance；tracks相当于pathways。这是常用的 paraphrase命题手法。 A、D两项所说实为同一问题，即time lags(时间间隔/滞后)所造成的困难，而这只是问题之一。B项所述文中未见。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>