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Comparatively few fresh-water species of fishes are limited in their distribution to a single river system, yet not many are fou

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问题     Comparatively few fresh-water species of fishes are limited in their distribution to a single river system, yet not many are found on both sides of a high mountain ridge, such as the Rocky Mountains in North America. That is to say, the fishes of the Mississippi Valley are generally different from those of the Pacific slope.
    While it is a well-known fact that the fish life in no two river systems, even though they empty into the sea on the same side of a divide(分水岭), is exactly identical, such streams do have many species in common. The principal rivers of the Atlantic slope of the United States, for example, contain several species common to all of them, including the bullhead catfish, the bluegill sunfish, and the largemouth bass. None of these species can endure salt water, so they cannot move from one river system to another. On the other hand, the more northern streams contain species not found in the southern ones, and vice versa. The common pike, for example, is found in the Atlantic streams from Maryland northward, and the brook trout and yellow perch occur only in the streams from North Carolina southward.
    How the present distribution came about must remain a matter of guess. It is quite probable that some of the streams, including those on opposite sides of a divide, may have been connected at one time. Again, streams may be entirely separate during normal weather, but an exceptionally heavy rainfall or the sudden melting of snow in the uplands sometimes causes floods which may form a temporary connection between them, providing a passageway for fishes. It is also possible that water birds may accidentally carry fish or spawn(卵)from one stream to another, or that man may be instrumental in such a transfer.
    Evidently, then, fresh-water fishes may become distributed far beyond the confines of the stream of their origin. Then chief factor in limiting the still wider distribution of species is temperature. This forms such an efficient barrier that comparatively few species of fresh-water fishes of the United States extend their range into Mexico. In Panama only one fish common to the fresh waters of the United States has been found, and that is the eel, which is not strictly a fresh-water form, as it enters salt water to spawn and is taken in fairly salty water at other times.
Species, which cannot tolerate saltiness, may spread into another river by the following mechanisms except______.
选项 A、being carried by birds
B、flooded connections
C、the actions of man
D、spawning in the river mouth
答案D
解析 细节题。第三段第三句提到突然暴发的洪水可能使山脊两边的河流连接起来。第四句提到水鸟也可能偶尔抓到鱼或它们的卵，然后带到另一条河流中去；或者人类也有可能促成这种转移。以上三种是对一鱼种出现在不同水域中原因的猜测。
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