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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.
Efforts to deliberately transplant the bluegill sunfish and the largemouth bass into Panama would probably fail due to______.
选项 A、new environment
B、saltiness
C、temperature
D、rainfall
答案C
解析 推理题。最后一段第二句指出限制鱼类分布扩大的主要原因是温度。这一原因成为美国淡水鱼向墨西哥、巴拿马等南美国家迁徙的主要障碍。
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