53. 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? (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. 53. For the hypothesis to be tenable it is important that the fish in streams in the Emerald River area that retain a wide temperature difference have not lost their ability to reproduce. Choice A asserts that these fish could still reproduce and is thus the best answer. Choice B undermines the hypothesis by suggesting a completely different hypothesis; choice C tends to support the claim that the temperature variation has lessened but does not show that this is the right explanation; since D relates a development after the native species began to decline, it does not bear on the hypothesis, which concerns the decline’s original cause; and choice E emphasizes the seriousness of the problem but sheds no light on what causes it.

为什么是A？怎么解释？

偶一开始做对是因为把signal理解成“妨碍”了，稀里糊涂地选了A，一看答案才发现不是我想得那样，结果就看不懂了

条件：建大坝后温差小了（50度减至6度），鱼没了

结论：是温差小是鱼少了的原因

加强方法：温差不小的地方（side stream）鱼就没有减少，用一个反面事实加

强了结论