Which of the Following Observations Was the MOST Crucial in the Development

Which of the following observations was the MOST crucial in the development of recombinant baculoviruses that can express large amounts of foreign proteins?

A) Baculoviruses can enter mammalian cells but can not replicate.
B) The polyhedrin gene is not essential for virus replication in cell culture.
C) Viruses missing the polyhedrin gene can be distinguished from wild-type viruses microscopically.
D) Recombinant viruses can produce large amounts of a foreign protein when introduced into insect cells in culture.
E) The polyhedrin gene has a very strong promoter that produces large amounts of mRNA late in the infection.