It can be difficult to definitively identify driver mutations in a tumor. Which of the following does NOT contribute to the difficulty of identifying driver mutations? a. Mutations in known proto-oncogenes and tumor suppressor genes may not be driver mutations.
b. A driver mutation must be definitively shown to be a loss-of-function mutation.
c. Driver mutations must be distinguished from passenger mutations.
d. The number of driver mutations in a tumor can be highly variable.