下列各组粒子中，电子层数和最外层的电子数都相同的是( ) A．O2-和Al3+

阅读理解。

     In formal paragraphs you will sometimes see a sentence at the end of the paragraph which summarizes

the information that has been presented. This is the concluding sentence. The concluding sentence is similar

to the topic sentence. They are both general sentences. The concluding sentence can be written in two ways:

     1. State the topic sentence in different words.

     2. Summarize the main points in the paragraph.

     You can understand concluding sentences with this example. Consider a hamburger that you can buy at

a fast-food restaurant. A hamburger has a top bun, meat, cheese, lettuce (生菜), and other elements in the

middle of the hamburger, and a bottom bun. Note how the top bun and the bottom bun are very similar. The

top bun, in a way, is like a topic sentence, and the bottom bun is like the concluding sentence. Both buns

"hold" the meat, onions, and so on. Similarly, the topic sentence and concluding sentence "hold" the supporting

sentences in the paragraph. Let's see how a concluding sentence might look in our sample paragraph about

Wheaton:

     My hometown is famous for several amazing natural features. First, it is noted for the Wheaton River,

which is very wide and beautiful. Also, on the other side of the town is Wheaton Hill, which is unusual because

it is very steep. The third amazing feature is the Big Old Tree. This tree stands two hundred feet tall and is

probably about six hundred years old. These three landmarks are truly amazing and make my hometown a

famous place.

     Notice how the concluding sentence, "these three landmarks are truly amazing and make my hometown

a famous place", summarizes the information in the paragraph. Notice also how the concluding sentence is

similar to, but not exactly the same as, the topic sentence.

     Usually, we begin a concluding sentence with "in conclusion" or "in summary". Remember, not all

paragraphs contain concluding sentences,especially if the paragraph is very short. However, if your paragraph

is very long, it is a good idea to use a concluding sentence.

1. What does the author mainly intend to tell us in this passage?

A. How to write a concluding sentence.

B. What the top bun and the bottom bun are like.

C. Why his hometown is a famous place.

D. To use a concluding sentence in our writing.

2. Why does the author write about a hamburger?

A. Because he can buy a hamburger at a fast-food restaurant.

B. Because a hamburger has a top bun and a bottom bun.

C. Because the top bun and the bottom bun are very similar.

D. Because he wants to explain his main idea in another way.

3. The underlined word "noted" probably means "_____".  

A. written down

B. famous

C. noticed

D. recorded

4. Which of the following statements is TRUE about the passage?

A. A sentence at the end of a paragraph is the concluding sentence.

B. All paragraphs contain concluding sentence.

C. It is a good idea to use a concluding sentence in a long paragraph.

D. The concluding sentence is the same as the to pic sentence.

5. The underlined phrase "These three landmarks" here refers to _____.

A. the Wheaton River,Wheaton Hill and my hometown

B. natural features,the Wheaton River and the Wheaton Hill

C. the Wheaton River,Wheaton Hill and the Big Old Tree

D. my hometown,Wheaton Hill and the Big Old Tree

铁、铬、锰及它们的合金称为黑色金属。

有两个关系R和S，分别包含15个和10个元组，则在R∪S，R-S，R∩S中不可能出现的元组数目情况是

A．15,5,10

B．18,17,7

C．21,11,4

D．25,15,0

贡献毛益率可通过（）来计算。

A.1-变动成本率

B.单位变动成本/单位售价

C.单位贡献毛益/单价

D.贡献毛益总额/销售额

E.固定成本/盈亏平衡点销售量

下列针对消防广播系统的说法中，不正确的是( )。

A．当建筑物的某层发生火灾时，为了利于确诊火情，组织人员有序疏散和灭火，推荐只对火灾层和相邻的上下各一层进行广播
B．扬声器的设置应能保证从本层任何部位到最近一个扬声器的步行距离不超过15m
C．为了更好地进行分散控制，扬声器可设置开关
D．建筑内背景音乐与火灾事故广播可以合用