What step is essential to the traditional production of flu vaccine? [A]Manufactur

What step is essential to the traditional production of flu vaccine?

[A] Manufacturers implant the vaccine into ordered chicken eggs.

[B] Scientists identify the exact strain soon after a flu pandemic starts.

[C] Public health measures are taken as an important pandemic-fighting tool.

[D] Viruses are deadened and made clean before being put into vaccine use.