已知当抛物线型拱桥的顶点距水面2米时，量得水面宽8米．当水面升高1米后，水面宽度是 （    ）米．

One of the difficulties in building an SQL-like query language for the Web is the absence of a database （） for this huge, heterogeneous repository of information. However, if we are interested in HTML documents only, we can construct a virtual schema from the implicit structure of these files. Thus, at the highest level of （） , every such document is identified by its Uniform Resource Locator (URL), has a （） and a text. Also, Web servers provide some additional information such as the type, length, and the last modification date of a document. So, for data mining purposes, we can consider the set of all HTML documents as a relation: Document(url, title, text, type, length, modif)Where all the （） are character strings. In this framework, an individual document is identified with a （） in this relation. Of course, if some optional information is missing from the HTML document, the associate fields will be left blank, but this is not uncommon in any database.

has a （） and a text

A．table

B．title

C． driver

D．event