在关系数据模型中，一个关系中的主码【 】。A．可以取值为NULL(空值) B．只

A省的M县奶牛场准备从B省N县引进一批奶牛，按照《动物防疫法》规定，A省的M县奶牛场必须首先取得（）动物卫生监督机构审批后，还应当经输出地动物卫生监督机构检疫合格。

本工程除氧器的形式为（）。

B

For a 400-year-old art form, operas had a bad fame: overweight actresses singing the words which are hard to understand in one of those romance languages you were supposed to learn in high school. And with tickets costing as much as $ 145 a performance, opera goers also have a certain appearance in people’s mind: rich, well-dressed and old.

But now opera companies around the country are loosening their ties and kicking off their shoes in an attempt to bring the opera to the audience. It needs to keep it alive, the young and not-so-rich.

Opera producers have found that to attract this crowd, they need to make the opera closer to common people. That means no formal suits, old-styled theatre or band-breaking ticket prices. And because young people don’t or won’t come to the opera, companies are bringing the opera to them, giving performances in such unusual places as parks, libraries and public schools.

The Houston Grand Opera’s choice is the public library, where it performs “mobile operas”, shortened versions (剧本) of child-friendly operas. This summer’s production is Hansel & Gretel. By performing smaller versions of large productions, producers are able to make people interested while keeping costs at a reasonable level. The San Francisco Opera, which will be celebrating its 75th anniversary this year, is staging Cinderella free of charge, keeping costs down by employing students from its Young Artists’ Training Program.   

45．Which is the main idea of this passage?

A．Opera is famous for its long history.                                  

B．Opera is only performed for rich people.

C．Young people are not interested in opera.                              

D．Opera companies are trying to keep opera alive.

46．The underlined phrase “loosening their ties and kicking off their shoes” (paragraph 2) means ______.

A．breaking up the old rules                      B．changing the formal suits

C．making the audience at ease                  D．advertising themselves

47．The San Francisco Opera Company employs student actors in order to ______.

A．celebrate its 75th anniversary                B．reduce the cost

C．attract young people                             D．make Cinderella popular

48．From the passage we can infer that ______.

A．the tickets for operas are very expensive                               

B．operas are performed in a difficult language

C．operas are not so popular an art form today                           

D．students enjoy performing operas very much

咨询方案的形成过程通常是建立对客观事实的（）基础之上。

A.感性分析

B.理性分析

C.实证分析

D.调查分析

线路设备状态评定应以千米为单位，满分为（）为失格。

A.100分

B.100～85分

C.85（不含）～60分

D.60分（不含）以下