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A cancer cell has a very high division rate even when no growth factors are present. Which of the following mutations, if any, could cause this? Select one: a. Again-of-function mutation in a growth factor receptor gene. O b. Again-of-function mutation in the gene encoding the retinoblastoma protein that regulates entry into the cell cycle. c. A loss-of-function mutation in a cyclin gene. d. None of the other answer choices are correct. e. More than one of the other answer choices are correct.