单选题 The basic causes are unknown although certain conditions that may lead to cancer have been ______.
A. identified B. guaranteed C. conveyed D. notified

【正确答案】 A
【答案解析】[解析] identify“识别，鉴别”；guarantee“保证”；convey“运送；传达”；notify“通知”。题句意为：“虽然导致癌症的某些条件已经鉴别出来，但是根本的原因还不得而知。”