Bird flu , a contagious disease of animals , is believed ____ by viruses that normally infect only birds and less commonly pigs , and rarely humans .
A.caused | B.being caused | C.to be caused | D.to have caused |
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Bird flu , a contagious disease of animals , is believed ____ by viruses that normally infect only birds and less commonly pigs , and rarely humans .
A.caused | B.being caused | C.to be caused | D.to have caused |