Which of the following has been a major technological problem with producing a vaccine to the H5N1 avian influenza?

A) It was difficult to produce a reassortant virus strain.
B) It was difficult to identify the most antigenic regions of the virus.
C) The virus can not be grown in cultured cells.
D) It was difficult to produce enough of the vaccine strain in chicken eggs.
E) The virus replicates to very low titers.