题目

.设集合，，则（    ） A.     B. C.     D. 答案：D 【解析】 【分析】 利用并集概念与运算即可得到结果. 【详解】∵A＝{1，2，3}，B＝{2，3，4}， ∴A∪B＝{1，2，3，4} 故选：D． 【点睛】本题考查并集及其运算，解题的关键是正确理解并集的定义及求并集的运算规则，是集合中的基本概念型题．阅读下列图文，简要回答所给问题。A growing number of families are sending children on study tours abroad.Some parents are worried that their children are too young to deal with any problems they might face during a trip. However, some believe they could take care of themselves and communicate with people well during the trip.A report shows that at least 800，000 overseas study tours were booked for Chinese children during the summer vacation. Each tour cost an average of 25，000 yuan($3，770) per person, and most were booked(预订) by middle?class families in large cities for pre?high school children, according to the report. About 13 percent were booked for children ages 3 to 6, while those ages 7 to 12 are 31 percent.Besides the United States and the United Kingdom, countries in Africa, South America and Southeast Asia are becoming popular in recent years. You can get more facts from the following charts.1.What are some parents worried about if they send their children on study tours abroad?___________________________________________________________ .2.Who were most of the study tours abroad booked for?___________________________________________________________ .3.What percent of the people want to experience different cultures through study tours abroad?___________________________________________________________ .4.How long do 20 percent of the people want to study abroad?___________________________________________________________ .5.Will you take a study tour in foreign countries? Why or why not?___________________________________________________________ .