问题 单项选择题

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

答案

参考答案:B

填空题

自然界有一种“雷雨发庄稼”的现象,即在雷雨过后,植物生长得更好。下面一段短文就是叙述雷雨中发生的一系列化学变化过程:

①空气中的N2在放电条件下与O2直接化合生成无色且不溶于水的一氧化氮气体;

②一氧化氮的密度比空气略大,不稳定,常温下就易与空气中的O2反应生成红棕色的二氧化氮的气体; ③二氧化氮气体有毒,易与水反应生成硝酸(HNO3)和一氧化氮;

④生成的硝酸随雨水洒到大地上,同土壤中的矿物质相互作用,生成可溶于水的硝酸盐。

请填写下列空白:

(1)上述短文中,描述二氧化氮化学性质的是                   (填序号)。

(2)实验室制取一氧化氮时,下列收集方法不适宜的是                                        。

A.向上排空气法 B.向下排空气法 C.排水收集法

(3)写出二氧化氮与水反应的化学反应方程式:                                                  ,其中氮元素的化合价在此反应中发生的变化是                   

A.不变 B.仅升高 C.仅降低 D.既有升高又有降低

(4)“雷雨发庄稼”,植物生长得更好的原因是                      

A.生成的一氧化氮可以作为氮肥 B.生成的二氧化氮可以作为氮肥 C.生成的硝酸盐可以作为氮肥

单项选择题