Passage 4Questions 76 to 80 are based on the following passage:
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（分水嶺）,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.
76.According to the passage,________.
A.The Mississippi River and the Pacific slope are on the same side of the Rocky Mountains
B.The Mississippi River and the Pacific slope are on the two sides of the Rocky Mountains
C.The Mississippi River and the Rocky Mountains are on the two sides of the Pacific slope
D.The Pacific slope and the Rocky Mountains are on the two sides of the Mississippi
77.The fishes in the Atlantic slope cannot move from one river system to another because __________.
A.there is no canal linking different rivers
B.fishes cannot swim by way of the Atlantic
C.different rivers do not have the same salt content
D.different fishes need different rivers
78.What is NOT the reason for the same fish to be found in different rivers?
A.Different rivers may have been linked to each other in the past.
B.Floods carried fish from one river to another.
C.Birds carried fish from place to place.
D.Earthquakes may have caused links between rivers.
79."man may be instrumental in such a transfer" in the last sentence means __________.