You are answering a USMLE-style medical question. Question: A 65-year-old male is treated for anal carcinoma with therapy including external beam radiation. How does radiation affect cancer cells? Options: A) Induces the formation of thymidine dimers B) Induces the formation of disulfide bonds C) Induces deamination of cytosine D) Induces breaks in double-stranded DNA Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.