Proto-oncogenes can change into oncogenes that cause cancer. Which of the following best explains the presence of these potential time bombs in eukaryotic cells?

A) Proto-oncogenes first arose from viral infections.
B) Proto-oncogenes normally help regulate cell division.
C) Proto-oncogenes are genetic "junk."
D) Proto-oncogenes are mutant versions of normal genes.
E) Cells produce proto-oncogenes as they age.