The rb gene encodes a protein that inhibits E2F, a transcription factor that activates several genes involved in cell division. Mutations in rb are associated with certain forms of cancer, such as retinoblastoma. Under each of the following conditions, would you expect cancer to occur?
A. One copy of rb is defective; both copies of E2F are functional.
B. Both copies of rb are defective; both copies of E2F are functional.
C. Both copies of rb are defective; one copy of E2F is defective.
D. Both copies of rb and E2F are defective.