Which of the following is the primary reason why it is difficult to produce packaging cell lines for both retrovirus and adenovirus vectors?

A) The proteins are degraded by the proteasome in the cell.
B) It is difficult to express high levels of the proteins.
C) Many of the required viral proteins are toxic to the cell.
D) The immune system recognizes the proteins and kills the cell.
E) The proteins stimulate the RNAi defenses in the cell.