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Two decades after the Emerald River Dam was built, none of the eight fish species native to the LACE w:st="on">LACENAME w:st="on">EmeraldLACENAME> LACETYPE w:st="on">RiverLACETYPE>LACE> 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.

怎么回事,没看懂题,答案A

修了大坝后，在大坝下游的鱼不繁衍了

发现修大坝后水温变化范围缩小为6度

所以科学家推测剧烈上升的水温可能刺激鱼的繁衍循环。

同意楼上。A意思是说：这些本土的鱼仍然能在大坝所在河的支流繁殖，而这些支流的年度温差仍然维持在50度左右。

即支持了原文结论的温差引起本土鱼消失。