填入横线处的句子，与上下文衔接连贯的一项是[ ]

         我蹲下来，背起了母亲，妻子蹲下来，背起了儿子。我的母亲虽然高大，然而很瘦，自然不算重；___________________，自然也轻；但我和妻子都是慢慢地，稳稳地，走得很仔细，好像我背上的同她背上的加起来，就是整个世界。

A、儿子毕竟幼小，虽然很胖

B、儿子毕竟很胖，即使幼小

C、儿子尽管很胖，毕竟幼小

D、儿子虽然很胖，毕竟幼小

One of the difficulties in building an SQL-like query language for the Web is the absence of a database (131) 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 (132) , every such document is identified by its Uniform Resource Locator (URL), and a (133) and a text. Also, Web severs 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, rifle, text, type, length, modif)
Where all the (134) are character strings. In this framework, an individual document is identified with a (135) 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.

A．relation

B．field

C．script

D．tuple