单选题某班级共50名学生，班级进行了一次摸底考试，分为语文，数学和英语三科，每名学生的成绩评定为A、B、C、D四个等级。其中，语文得到A级的学生人数为23人，数学得到A级的人数为21人，英语得到A级的人数为27人，有18名学生同时拿到2个A级，只有5名学生三科成绩均为A级。那么该班学生中，有多少人三科成绩均未获得A级？（）A 7B 8C 9D 10

Class Type:ReadingClass
TeachingContents:The lesson mainly talks about the ThreeGorges Dam, including its features, advantages and disadvantages.
Teaching Objectives:
Knowledge objective
Students can get more information about the features, advantages anddisadvantages of the Three Gorges Dam.
Abilityobjectives
(1)Students can understand the main content and detail information ofthe passage through fast reading and detailed reading.
(2)Students can introduce the Three Gorges Damin their own words.
Emotional objective
Students can develop a sense of national pride through learning thispassage.
Teaching Key and Difficult Points:
Studentscan get a general idea of the passage and learn to use reading strategies tounderstand the material, such as summarizing, skimming and acquiring usefulinformation
Major Steps:
Step1 Pre-reading (5 minutes)
1. Show a poem of Mao Zedong on PPT:Walls of stone to hold back clouds and raintill a smooth lake rises in the narrow gorges.Students guess which placeit is. Then show some beautiful pictures of the Three Gorges Dam.
2. Students have a discussion: Why did weChinese build the Three Gorges Dam Then ask some of them to share their ideas.
(Justification: The poem and pictures canarouse students' interest in this class. Discussion will stimulate students’ backgroundinformation and students will get prepared for the reading class, and it willbe helpful to understand the material. )
Step 2 While-reading (9 minutes) .
1. Fast reading: Task 1 : Students read the passage quickly and give asummary of each paragraph. Students share their ideas and help students toanalyze the article structure.
2. Careful reading: Task 2: Students read the passage again and completethe following table. Then ask some students to show their answers and giveassessments.



Task3 : Students pay attention to Paragraph 2-5, and work in pair s to discuss thequestions below:
(1) What's the biggest advantage of the Three Gorges Dam project
(2) What has happened to the historical relics near the Three GorgesDam
Ask some pairs to show their opinions and lead students t0 1eam thenecessity of the Three Gorges Dam.
(Justification: Through three tasks, students can have a thoroughunderstanding of the passage, and improve reading skills by using different readingstrategies: such as summarizing and acquiring useful information.)
Step 3 Post-reading (6 minutes)
Interview: students work in a group of three. Two students act asengineers from the Three Gorges Dam and one student as a reporter. The reporterasks some questions about the Three Gorges Dam and their feelings about the constructionproject. After three minutes, invite some groups to show their dialogues.
(Justification: Students can improve their speaking ability by talkingabout the Three Gorges in their own words and develop a sense of national pride.)

三科总和为93×3= 279；语文、数学总共180分，那么语文、英语总成绩187，所以英语为279-180=99；数学为 279-187=92；所以语文为88分。故答案为A。

描述统计；差异量数。 CV(语文)=1.2÷88×100%=1.36；cv(数学)=1.4÷93×100=1.51%。通过比较差异系数可知，数学成绩的分散程度比语文成绩的分散程度大。

标准分数的计算公式为 {图0}其中x为某一具体分数，μ是平均数，σ是标准差。即由计算可得，小明的语文、数学和英语三科成绩转换为标准分数后分别是1．5、1和2。

全面发展，尊重个性。素质教育要求使学生获得全面发展。

除3名成绩在60分以下的学生外，其余成绩均在75—95分之间，75～95共有21个不同的数，将这21个数作为21个抽屉，把47—3=44个学生作为物品。44÷21=2……2根据抽屉原理，至少有l个抽屉至少有3件物品，即47名学生中至少有3名学生成绩是相同的。

描述统计；相对量数；标准分数。 用Z分数的定义公式就能算出来，标准差是6。

学习成绩为优的学生数量=各年级学生数量×10%;学习成绩为差的学生数量=各年级学生数量×30%;由以上信息可知某年级学习成绩为优的学生数量及学习成绩为差的学生数量和该年级学生总数量成正比。高二年级成绩为优的学生多于高一年级成绩为优的学生,可得高二年级学生总数多于高一年级学生总数,进一步可得高二年级成绩为差的学生数量多于高一年级成绩为差的学生数量。考生容易根据惯性思维错选A选项,注意题干并没有假设高一年级和高二年级学生总数量相同。

本题考查的是两个相芙样本对应的总体方差之间的差异显著性检验。此时需用下列公式进行t 检验：

分别为两个样本方差，r为两个样本之间的相关系数，凡为样本容量，自由度为n-2。