在《The English Noun Phrase：The Nature of Linguistic Categorizatio》这本权威的语法书里，出现了这样两个句子，作者认为完全符合语法，请问这是什么语法。

a. The Borough of Chelsea and Kensington is a very wealthy one.

a1. Chelsea and Kensington is a wealthy borough.

比较
b. The Boroughs of Chelsea and Kensington are very wealthy.

为什么a和a1的be动词是单数形式？

    【答】a. The Borough of Chelsea and Kensington is a very wealthy one. 中的The Borough 是单数，作主语， Chelsea and Kensington 是它的同位语，所以谓语动词用单数。

     a1. Chelsea and Kensington is a wealthy borough. 中的Chelsea and Kensington 是一个整体，是一个borough，做主语，所以后面的be动词用单数形式is。