摘要：33．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
查看习题详情和答案>>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