Productive Infections with Human Papillomaviruses Have Been Difficult to Study

Productive infections with human papillomaviruses have been difficult to study in cultured cells.Which of the following explains why?

A) They only infect human cells in culture.
B) They only produce a few virions per infectious cycle.
C) The viral particles are difficult to purify.
D) They only produce virions in differentiated keratinocytes.
E) They induce a strong interferon response in infected cells.