题目解析:
以natural selection做关键词定位至第三四句: Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. 说自然选择会选择那些最容易寻找寄主的寄生者,同时也会选择寄主的防御能力,所以答案是B。A的比较,C的toxic material和isolate,还有D的大小均没有相关信息。