This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': "Excessive ultraviolet [uv) radiation is harmful to life'. The damage caused to the biological systems by ultraviolet radiation. \nSelect and explain. \nA. Inhibition of DNA synthesis, B. Formation of thymidine dimmers, C. Ionization, D. DNA fragmentation", 'role': 'user'}, {'content': 'Answer: B \nExplanation: UV rays causes DNA damage by forming TT dimers (pyrimidine dimmers) which prevents replication', 'role': 'assistant'}]