Which of the following is true about the present swine flu viruses？
A. The viruses can infect different species.
B. The genes of the viruses come from pigs.
C. The viruses mix genes from different species.
D. The genes of the viruses were made artificially.
A．cameB．comeC．becomeD．became
A. came
B. come
C. become
D. became
______ that a society like the National Society for the Prevention of Cruelty to Children
A. How does it come
B. How it comes
C. How did it come about
D. How did it come
According to the passage, which of the following is true about human-generated air pollution in localized regions？
A. It can be dwarfed by nature's output of pollutants in the localized region.
B. It can overwhelm the natural system that removes pollutants.
C. It will react harmfully with naturally occurring pollutants.
D. It will damage areas outside of the localized regions.
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