Ants are probably the most frequent and certainly the most persistent defenders of plants.[A]Since the highly active worker ants require a great deal of energy,plants exploit this need by providing extrafloral nectar that supplies ants with abundant energy.<B>To return this favor,ants guard the nectaries,driving away or killing intruding insects that might compete with ants for nectar.[C]Many of these intruders are herbivorous and would eat the leaves of the plants.[D]
此时我们需要注意的，将句子插入原文后，插入后的句子不能改变原文间的内在逻辑关系。
如果我们放在A处，那么插入后的文章给人的感觉就比较突兀，因为句话与第二句话没有铺垫，联系不够紧密。而如果放在B处的话，前边的plants supply ants with abundant energy和后边的to return this favor的内在联系就被打破了，同样也是不可取的。那么如果放在C处，句子的主语并没有改变，而且driving away or killing intruding insects和capture the insects也是有关联的。因此C处是合适的。而如果放在D处，那么intruders和插入句中的insects是同一回事，可是句子的主语却是they。因此句子的逻辑就不对了，所以不能插入在D处。
