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

D.They have to predict which strains of flu will become active months before the flu season begins.

The text states, "Creating a flu vaccine is a bit of a guessing game, because the shots take months to manufacture. Each year in February, the World Health Organization (WHO) tries to predict what viruses may be circulating the following winter, a process that can change as bugs mutate. " This supports the answer that a challenge for vaccine makers is determining which strain of the flu must be targeted in order to prevent its spread.