You are a fourth year medical student midway through your radiation oncology elective. Today has been rather quiet, so it has given you a chance to corner your favourite radiation oncologist (Dr. Ruffledrop), and chat a bit. Dr. Ruffledrop has been quite pleased with your progress, but wants to know if you really understand the fundamentals of radiation oncology. She asks you which phase of the cell cycle is easiest to cause a double stranded DNA break with radiation. You answer: