题目

Scientists are working to stop a deadly virus that infects(感染)pigs. The virus is causing huge losses for farmers in China and other Asian countries. Many farmers have tried to contain African swine flu by quarantining(隔离)pigs and destroying infected animals. But the spread of the disease into East Asia has stepped up the search for other ways to deal with the problem. The virus had not been considered a major concern for researchers until it appeared in China last year. Chinese farmers raise about half of the world's pigs. Some researchers think the virus might have come from infected Early this year cruel snowstorm hit southern China, ______ great transportation problems in some areas. 　 A. causing　 B. caused　 C. to cause　 D. having caused