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All your cells contain proto-oncogenes, which can change into cancer-causing genes. Why do cells possess such potential time bombs?
A. Cells produce proto-oncogenes as a by-product of mitosis.
B. Proto-oncogenes are necessary for the normal control of cell growth and division.
C. Proto-oncogenes protect cells from infection by cancer-causing viruses.
D. Proto-oncogenes are unavoidable environmental carcinogens.
E. Proto-oncogenes are genetic junk that has not yet been eliminated by natural selection.