You are answering a USMLE-style medical question. Question: A 77-year-old woman presents to the emergency room with the complaints of fever, malaise, and night sweats. She recently observed an enlargement of her axillary lymph nodes, which she examines on a weekly basis. She has a remote history of breast cancer in her 60s that was treated with radiation and chemotherapy. She also reports a history of extensive travel to Africa and a 30-pack-year history of smoking. On physical exam, several axillary lymph nodes are palpable with a large non-tender palpable mass in her right axilla measuring 10 x 8 cm. Fine-needle aspiration demonstrates what the pathologist describes as "a centroblastic and immunoblastic cell presence, suspicious for non-Hodgkin’s lymphoma (NHL)–diffuse large B cell variant". Which of the following risk factors is responsible for this patient’s condition? Options: A) Travel to Africa B) Axillary lymph node involvement C) Previous radiation therapy D) Previous breast cancer Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.