Read the following passage and choose the correct answer to each of the questions. Scientists have established that influenza viruses taken from man can cause diseases in animals. In addition, man can catch the diseases from animals. In fact, a greater number of wild birds seem to carry the viruses 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 earth 100 million ...

Read the following passage and choose the correct answer to each of the questions.

Scientists have established that influenza viruses taken from man can cause diseases in animals. In addition, man can catch the diseases from animals. In fact, a greater number of wild birds seem to carry the viruses 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 earth 100 million years and is able to carry the viruses without contracting the disease. There is even convincing evidence to show that virus strain 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 recombination is a great variety of strains containing different H and N spikes. This raises the possibility that a human influenza viruses can recombine with an influenza virus from a lower animal to produce an entirely new spike. Research is underway to determine if that is the way 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.

According to the passage, scientists have discovered that influenza viruses _______