【正确答案】 B

【答案解析】[解析] 本题考查语义推断。第二段第一句：“The nature of viruses made them impossible to detect for many years，even after bacteria had been discovered and studied．(即使细菌被发现并被研究之后，病毒的特性也使它们很多年来不可能被察觉)。”由此可知nature意思为“特性”， [B]characteristics与之相近。