Many Tumor Cells Are Able Divide Uncontrollably in Cell Culture

Many tumor cells are able divide uncontrollably in cell culture in the laboratory. Some of these cell lines have genomes with a mutant tumor-suppressor gene. Adding a wild-type copy of this tumor-suppressor gene to the genomes of these cells reduces their ability to divide in culture. In contrast, adding a wild-type copy of a proto-oncogene to the genomes of a tumor cell line with an activated oncogene doesn't have any effect on the uncontrolled cell division shown by this line. Which of the following statements BEST explains why adding a wild-type proto-oncogene doesn't affect cell division in these cell lines?

A) Most point mutations that produce activated oncogenes are dominant gain-of-function mutations.
B) Many proto-oncogenes encode proteins that act to promote apoptosis.
C) Many proto-oncogenes encode proteins involved in DNA repair.
D) Many proto-oncogenes are inherited as recessive mutations.
E) Most protein products of proto-oncogenes don't have a role in cell division.