There are many trees on ()side of the road,and ()of the trees is still increasing

题目

There are many trees on ()side of the road,and ()of the trees is still increasing
第一个空为什么不能填either?

答案

第一空：each/ either 应该可以呀,因为each/ either 后接单数名词,指两者中任何一个,第二空：all,指所有的树