Evidence that the development of cancer is a multistep process includes:

A)the observation that certain tumor-suppressor genes and oncogenes are involved in a sequential manner in the development of colon cancer.
B)the fact that proto-oncogenes are widely conserved in evolution.
C)the usual occurrence of retinoblastoma at a young age.
D)the development of a cancer as a result of activation of a single oncogene by any of a variety of mechanisms.
E)the fact that there are many genetic and epigenetic mechanisms that lead to inactivation of the same tumor-suppressor gene.