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 LACE w:st="on">LACENAME w:st="on">EmeraldLACENAME> LACETYPE w:st="on">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.

答案是A，我是按照无关选项的方法排除其他的，但是仍然不明白在大坝下支流中幸存的鱼群怎样支持了原文中的假设？

还请NN指教

文章说的是温度下降了,鱼不繁殖了,科学家假设的是温度的升高是对鱼重新繁殖的一个信号.而A说这种鱼在50度的时候是可是繁殖的,说明了温度是影响鱼繁殖的因素.