Every year, a new influenza vaccine is developed. What is the most likely reason that doctors do not use the same vaccine each year? A. A new, different virus that causes influenza enters the food chain through plants about once every year, infecting humans. B. The vaccine turns into a new virus that causes a recurrence of the same influenza each year. C. Any vaccine can only last for a year before it dies. D. The virus which causes influenza adapts each year to the last vaccine in order to survive and reproduce.