【答案解析】For the newly named DP, the component D functions as the head of the whole construction, which is realized in the definiteness (Def.) and agreement(Agr.) between the noun and its modifier (such as "the" or "a", nouns in singular form or in plural form). Therefore, as this new name indicates, the N/NP in this construction then becomes the complement of the head D.
This to some extent helps us English learners when we are confronted with the rules concerning the agreement between the noun and the article, definite or indefinite. When the noun is in its singular form, there is usually a determiner to modify it, while it is optional for a plural noun to have a determiner to precede it. The use of the definite determiner "the" is different from that of the indefinite one "a/an". Therefore, we could be easier to tell expressions like "this books" and "those book" are impossible in English, since in the construction, it is ensured by the property of D that the pre-modifier and the noun should be in agreement in number.
This renaming of NP to DP is significant in that it makes a clear labour division between the component D and the NP component, resembling the way the clauses are structured. In IP (the clausal structure), a functional category I is the head of the clause. Similarly, the component D, as the head of DP, governs the correct form of the NP considering the components outside the NP. For example, Agr. illustrates the agreement of the NP to a bundle of features such as gender, number, person. What"s more, by separating the component D, it shows the common feature of nominal phrases in English, that is, the form of the noun should agree with its modifier in number, gender, and person. Consequently, it makes analysing the deep structure of the nominal phrases easier.