Which of the following is true regarding tumor-inducing retroviruses?

A) Retroviruses are double-stranded DNA viruses.
B) The enzyme reverse transcriptase is essential for transcribing the viral DNA into RNA.
C) Cancer-causing genes encoded by the viral genome are called proto-oncogenes.
D) Each type of viral gene that can cause cancer can potentially regulate the expression of cellular genes.
E) Viral genes that can cause cancer are different from other viral genes because they possess introns.