What does the author say about the flu B virus? A. B Virus is relatively stable and causes mild infections that do not spread far through the population. B. B Virus is unstable, and is responsible for the epidemics that cause frequent concern. C. B virus is extremely unstable and is responsible for some of the worst outbreaks of the disease. D. B virus has a very developed variability, and it affects only human beings. 【
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