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.