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  3. Two decades after the Emerald River Dam was built, none of the eight fish species native to the Emerald River was still reproduc

Two decades after the Emerald River Dam was built, none of the eight fish species native to the Emerald River was still reproduc

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问题 Two decades after the Emerald River Dam was built, none of the eight fish species native to the Emerald River was still reproducing adequately in the river below the dam. Since the dam reduced the annual range of water temperature in the river below the dam from 50 degrees to 6 degrees, scientists have hypothesized that sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle. Which of the following statements, if true, would most strengthen the scientists’ hypothesis?
选项 A、The native fish species were still able to reproduce only in side streams of the river below the dam where the annual temperature range remains approximately 50 degrees.
B、Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were critical breeding areas for the native species of fish.
C、The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas the lowest recorded temperature of the river after the dam was built has been 43 degrees.
D、Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing with the declining native fish species for food and space.
E、Five of the fish species native to the Emerald River are not native to any other river in North America.
答案A
解析 要说明该假说是可靠的，证明Emerald河流域中温度差别较大的河水中生长的鱼没有失去它们的繁殖能力就很重要，选项(A)说明这些鱼仍能繁殖，因此是正确答案；选项(B)通过提出一个完全不。司的假说削弱了该假说；选项(C)试图支持温度变化减小了的说法，但没有证明这是正确的解释；由于(D)谈到土产鱼类开始减少后的一种发展，同关心这种减少的最初原因的该假说无关；选项(E)强调了该问题的严重性，但没有说明是什么原因造成的。
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