单选题 The problem with normal flu vaccines is that influenza viruses are notorious "shapeshifters," says Dr. Jay Butler, director of the CDC's H1N1 Vaccine Task Force. They mutate (突变) rapidly and can even exchange genes with other strains of flu. However, federal officials knowfrom monitoring H1N1 in the Southern Hemisphere this summer that the virus has been unusually stable. Therefore the immune response generated by the vaccine should match the circulatingstrain of swine flu. "The vaccine will be effective," says Butler.
Federal officials have found that H1N1 virus

• A. notorious "shape shifters".
• B. has been very stable.
• C. has mutated rapidly.
• D. has exchanged genes with other viruses.