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  3. Countering their expectations, biologists working in Bangladesh have found a thriving population of 6,000 Irrawaddy dolphins, a

Countering their expectations, biologists working in Bangladesh have found a thriving population of 6,000 Irrawaddy dolphins, a

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问题     Countering their expectations, biologists working in Bangladesh have found a thriving population of 6,000 Irrawaddy dolphins, a species restricted to brackish bays and rivers from southern Asia to northern Australia that marine mammal experts had worried was vulnerable to extinction.
    The population, many times larger than any other known regional groups of the dolphins, was revealed in 2004 in the first systematic survey for marine mammals along Bangladesh ’ s coast of waterways, bays and mangrove-fringed islands. The full results were described Wednesday in Hawaii at the first international conference on protected areas for marine mammals and in a paper in the winter issue of the Journal of Cetacean Research and Management.
    American and Bangladeshi biologists conducted the dolphin survey by boat. The researchers said that the six to eight-foot dolphins, while apparently thriving, needed to be protected and monitored in view of rising threats like entanglement in fishing nets, a decline in freshwater flows because of dam construction and inland diversions of water along the rivers that sustain the coastal ecosystems.
    The scientists also signaled a long-term threat to the dolphins from global warming, which climate studies project will raise sea levels and change the river flows as Himalayan glaciers erode. This would shrink the species’ range, which is restricted to water with low salinity.
    The Wildlife Conservation Society, which led the study, is working with the Bangladesh Ministry of Environment and Forests to create protected areas for the dolphin and for another species, the Ganges River dolphin, and seeking money for the effort, said Howard Rosenbaum, a biologist who directs the "ocean giants" program of the nonprofit group. "This mother-lode population in Bangladesh really gives us hope for the survivability of the species in the long term," Dr. Rosenbaum said.
    Dolphin and porpoise species that have adapted to rivers and deltas around the world have long been considered some of the most vulnerable of marine mammals because of their restricted habitats. In 2007, experts concluded that the baiji, a river dolphin that thrived in the Yangtze River for 20 million years in what is today China, had been driven extinct by a variety of activities by the nearly half billion people now living in that watershed.
    The vaquita, a porpoise living in waters where the Colorado River empties into the Gulf of California, is critically endangered, biologists say, depleted by fishing nets and the disruptions in the river’s flow from dam construction.
Biologists called on urgent protection of dolphins on account of ______.
选项 A、the widespread distribution of fishing nets
B、the excessive dam construction along rivers
C、the rising human threat due to inconsideration
D、the deteriorated and ruined coastal ecosystems
答案C
解析 根据第三段中的“…the six-to eight—foot dolphins…needed to be protected and monitored inview of rising threats…”，这些海豚需要保护，主要因为人类各种考虑不周全的行为对其威胁日益增长。所以，C应为答案。
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