问题 选择题

【理科】将标号为1,2,…,10的10个球放入标号为1,2,…,10的10个盒子里,每个盒内放一个球,恰好4个球的标号与其在盒子的标号不一致的放入方法种数为(  )

A.2520

B.1890

C.210

D.9

答案

根据题意,分2步进行,

先从10个小球中取出4个,将剩余的6个放入对应的盒子里,有C104=210种情况,

再分析选出4个球的标号与其在盒子的标号不一致的情况数目:

假设取出的是1、2、3、4个小球,剩下的是1、2、3、4号盒子,

对于1号球,有3种放法,假设放到了2号盒子中,则2号球也有3种放法,3、4号球只有1种放法,则有3×3=9放法,

则恰好4个球的标号与其在盒子的标号不一致的放法有210×9=1890种,

故选B.

单项选择题
单项选择题

There (1) not one type of reading but several according to your reasons for reading. To read carefully, you have to (2) your reading speed and technique (3) your aim (4) reading. Skimming is a technique necessary for quick and efficient reading.

When skimming, you (5) the reading (6) quickly in order to get the (7) of it, to know how it is organized, (8) an idea of the tone or the intention of the writer. Skimming is (9) an activity which (10) an overall view of the text and (11) a definite reading competence.

Skimming doesn’t need reading all the material, but it doesn’t mean that it is an (12) skill for the lazy, because it need a high degree of alertness and concentration.

When you read, you usually start with (13) understanding and move towards detailed understanding rather than working the other way round. But (14) is also used after you have already carefully studied and you need to (15) the major ideas and concepts.

In order to be able to skim quickly and (16) through a text, you should know where to look for what you want. In preview skimming you read the introductory information, the headings and subheadings, and the summary, if one is provided. (17) this skimming, decide whether to read the material more thoroughly, and select the appropriate speed (18) you read.

The same procedure (19) for preview skimming could also be used to get an overview. Another method would be to read only key words. This is done by omitting the unnecessary words, phrases, and sentences.

In order to skim efficiently and fulfill your purpose, (20) practice is necessary.

12().

A.awful

B.unsuitable

C.ideal

D.wrong