【正确答案】正确答案：This founding firstly demonstrates one point that the acquisition of auxiliary verbs comes later than that of comparatively "simpler" morphemes such as pronouns. Therefore, it is unlikely for children to make mistakes as shown in e). The reason for this phenomenon could be explained by the fact that in the early stage of children's language acquisition, the speech at first only contains content words and lacks the function elements. This founding also shows that children have a certain process of learning the negative form. There are generally three stages. (1) At first, the negative element is not part of the structure of the sentences. It is simply attached to the beginning, as shown in d). (2) The negative element is inserted into the sentence. The additional negative forms don' t and can't are used, and with no and not, begin to be placed in front of the verb rather than at the beginning, as shown in c). (3) Children begin to produce the appropriate part of do, be or the model verbs, to suit the person or tense, as shown in b) and a).