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Butterflies are among the most colorful insects, and their extraordinary diversity has been the focus of studies by amateur natu

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问题       Butterflies are among the most colorful insects, and their extraordinary diversity has been the focus of studies by amateur naturalists and professional entomologists for over two centuries. Butterflies have long been model organisms for basic biological research in the fields of behavior, population genetics, and sys-tematics. More recently, studies of butterfly life cycles in tropical rain forests have resulted in a cottage industry: "farms" or "ranches" where live butterflies are reared under controlled conditions to provide a supply of pupae for exhibits of live insects. Live butterfly pupae are sold to museums and zoos for public education and enjoyment such as in the academy’s newest exhibit featuring tropical butterflies that opens October 12.
     How do butterfly ranches and farms operate? One pioneering project developed by the government of Papua New Guinea encouraged farmers to take part in commercial exploitation of a renewable resource--insects. In the 1970s, government officials became aware that expatriates involved in the commercial collection of insects were enlisting the aid of local farmers who lived adjacent to rainforest habitats. The farmers were paid a small fraction of the market value of the specimens. The government was determined to change this state of affairs to ensure the locals received a larger share of the profit. So the expatriates were expelled from the country, and the Insect Fanning and Trading Agency was born. Most of the trade managed by the IFTA consists of wild-caught insects from rain forests.
     In the case of wild-caught butterflies, the majority captured by IFTA collectors are males due to behavioral differences between the sexes. (Males aggregate along river banks to imbibe sodium and are easier to capture.) Because males mate more than once, as long as the harvest of wild populations has the expected skewed sex ratio, the impact of collecting is minimal relative to naturally occurring predation. Now the IFTA controls all trade in insect specimens and has fostered a program of butterfly ranching to encourage protection of remnant rainforest habitats in the vicinity of a farm or village.
     The basic principle behind butterfly ranching is diversification of crops in a small garden plot to include the host-plants on which caterpillars feed. Only a small fraction of these larval host-plants are known to science. The main "research and development" activity of butterfly ranchers is discovering unknown host-plants through careful observation of egg-laying female butterflies. Once the host of a particular species is known, it can be cultivated in the garden near the edge of the rain forest to attract egg-laying females. If the natural concentration of a particular plant species is one per square mile of rain forest, and a farmer plants several dozen of the species, the result is a dense concentration of caterpillars in a short time. Butterfly ranching thus involves artificial manipulation of the natural density of caterpillar host-plants.
      Additionally, flowering plants, such as Hibiscus and Lantana, attract female butterflies, which need the nectar to renew their energy reserves after a bout of egg-laying. After the caterpillars mature, they are harvested and removed to cages for pupation. The IFTA sends out advisors to instruct villagers in butterfly husbandry techniques. Many farmers have learned that they can sell butterflies to supplement their income if they protect the remnant patches of rain forest that form a reservoir for this renewable resource.  
In Line 6 Para. 2, "this" refers to ______ .
选项 A、the expatriates came to collect insects in the rainforest neighborhood
B、the government developed a pioneering project
C、the expatriates involved in commercial exploitation of the insects
D、the expatriates made profit from commercial exploitation with the aid of rainforest habitats
答案D
解析 语义理解题。根据代词在文中的指代，内容通常为已经提及的事物，根据this前面内容“expatriates involved in the commercial collection of insects were enlisting the aid of local farmers who lived adjacent to rainforest habitats．”即从事昆虫商业化采集的侨民正在请求住在雨林附近的当地农民对他们提供帮助，可知答案为D 。其余为干扰项。
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