1、Sentence B is structurally more complex and more difficult for young learners to learn.
因为现在进行时要多一个助动词.
2、The second one was from the older child.
第一句是祈使句,第二句是陈述句.小孩说祈使句早于说陈述句.
3、"Where" is right."By" is wrong.
where是关系副词,指代先行词the process,引导定语从句a child uses one word like ball to refer to an apple,an egg,a grape and ball is overextension,并在从句中作地点状语.
by是介词,引出动作的执行者.如果选by,那么a child uses one word like ball to refer to an apple,an egg,a grape and ball is overextension就与主句失去了联系.