[单项选择]Scientists have not yet determined ______.


A. all of the effects of pollution on the human body
B. how pollution can be controlled successfully
C. when the atmosphere first became polluted
D. how to wash the black snow clean

[单项选择]Scientists have not yet determined ______.


A. all of the effects of pollution on the human body
B. how pollution can be controlled successfully
C. when the atmosphere first became polluted
D. how to wash the black snow clean

[单项选择]Scientists wilt have to ______ new methods of increasing the world's food supply in order to feed more hungry people.
A. catch up with
B. come up with
C. keep up with
D. put up with

[单项选择]

Scientists 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 of wild birds seem to carry the virus without showing an evidence of illness. Some scientists concluded 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. There is even convincing evidence to show that virus strains are transmitted from place to place and from continent to continent by migrating birds.
It’s 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 virus can recombine with an influenza virus from a low animal to produce an entirely new s
A. H and N spikes are produced
B. animal and bird viruses are combined
C. dissimilar types of viruses recombine
D. two viruses of the same type are contracted