Scientist have established that influenza viruses taken from man can cause the disease in animals.In addition,man can catch the disease from animals.In fact,a great number ofwild birds seem to carry the virus without showing any evidence of illness.Some scientists conclude that a large family of influenza viruses may have evolved in the bird kingdom,a group that has been on the earth 100 million years and is able to carry the virus without contracting the disease.These is even convincing evidence to show that virus strains are transmitted from place to place and from continent to continent by migrating birds.
　　
　　It is known that two influenza viruses can recombine when both are present in an animal at the same time.The result of such recombinations is a great variety of strains contraining different H and N spikes.This raises the possibility that a human influenza virus can recombine with an influenza virus form a lower animal to produce an entirely new spike.
　　
　　Research is underway to determine if that is the way that major new strains come into being. Another possibility is that two animal influenza strains may recombine in a pig, for example, to produce a new strain which is transmitted to man.
　　
　1. According to the passage,scientists have discovered that influenza viruses_____.
(A) cause ill health in wild birds
(B) do not always cause symptoms in birds
(C) are rarely present in wild birds
(D) change when transferred from animals to man

　2. What is known about the influenza virus?
(A) It was first found in a group of very old birds.
(B) All the different strains can be found in wild birds.
(C) It existed over 100 million years ago.
(D) It can survive in many different places.

　3. According to the passage, a great variety of influenza strains can appear when_____.
(A) H and N spikes are produced
(B) animal and bird viruses are combined
(C) dissimilar types virus recombine
(D) two viruses of the same type are contracted

　4. New strains of viruses are transmitted to man by_____.
(A) a type of pig
(B) diseased lower animals
(C) a group of migrating birds
(D) a variety of means

　5. It can be inferred from the passage that all of the following are ways of producing new strains of influenza viruses EXCEPT _____.
(A) two influenza viruses in the same animal re combining
(B) animal viruses recombining with human viruses
(C) two animal viruses recombining in one animal
(D) two animal viruses recombining in a human