What challenges do vaccine makers face in their attempts to produce an effective vaccine against the flu?A. Workers like having an excuse to miss work, so they try to prevent a good vaccine from being made. B.They have to predict which strains of flu will become active months before the flu season begins. C.People tend to resist getting shots, so the majority of the population does not get vaccinated. D.They cannot hire the most qualified employees because there is so little profit in making vaccines.