题目

 下列有关“一定”的说法正确的是    ①生物的生殖细胞中含有的全部染色体一定就是一个染色体组  ②经花药离体培养得到的单倍体植株再经秋水仙素处理后一定得到纯合子 ③体细胞中含有两个染色体组的个体一定是二倍体 ④体细胞中含有奇数染色体组的个体一定是单倍体  A．全部正确  B．①④  C．③④   D．全部不对 Junior school students in Harbin，Heilongjiang，had a "long weekend"．They had four days off from Oct．19-22．It was not for any festival，but was caused by serious smog． The smog was caused by high levels of PM 2．5 in the air． PM 2．5 is small particles（微粒） of pollution in the air with a diameter （直径） of 2．5 micrometers or less．Waste gas from cars and factories is the biggest cause of PM 2．5 pollution． On Oct．17，the World Health Organization named air pollution a cause of cancer． To better protect children，cities are taking action． On Oct．21，Harbin Education Bureau decided to close primary and junior high schools for two days．     Beijing started a program on Oct．22 which has a warning system with four levels．The red warning is the highest warning level．It means that the next three days will see serious pollution． When a red warning is issued（发布），schools will close． If there's pollution that's not as serious，the Beijing Commission of Education will ask schools to stop PE classes and outdoor activities． How will students be informed? "First，the Commission of Education will inform schools online，" Ye Yan，head of Beijing Shijingshanqu Experimental Primary School，told Beijing Times．"Then teachers will text students’ parents．" Will the days off affect the study plan? "Missing school days will be made up during the summer or winter vacations，"  said Wang Hao，a physical education teacher in Beijing Chaoyang district． For students in Harbin who had the long weekend，they would go to school on the next two Saturdays． 根据材料内容选出最佳选项 1 ．Why did students in Harbin have a long weekend?      A． Because the weather was extremely cold．     B． Because a special festival came．      C．Because the winter vacation began．           D．Because there happened serious smog． 2．Which of the following is TRUE about PM 2．5?      A．It makes water polluted．                               B．People can see it with eyes．      C．It only comes from factories．                         D．It can cause serious diseases． 3．When a red warning is issued in Beijing，what will happen?      A．All of the work will be stopped．                      B．People aren't allowed to go outside．      C．Schools will be closed for a few days．             D．Air pollution will be seen in a few weeks． 4．What does the underlined word "text" mean in Chinese? A．写信               B．发信息            C．写文章            D．发邮件 5．What is the reading mainly about?      A．What PM 2．5 is made up of．                        B．How people fight against air pollution．      C．What influence the smog brings to school．              D．How schools deal with the missing school days．