单选题 The basic causes are unknown, although certain conditions that may lead to cancer have been ______.
【正确答案】 A
【答案解析】[解析] 题干：致癌的基本原因还不清楚，尽管某些致癌因素已经得到确认。identify识别，认出；guarantee保证；notify通知；convey传导。根据题意，本题选A。