og－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.A

(E) Five of the fish species native to the Emerald
            River are not native to any other river in North America.

B undermines the hypothesis by suggesting a completely different hypothesis;

请帮忙看一下B的解释，不是很能理解？什么他怎么个削弱法？