13.D【解析】4~10年时λ<1,种群数量减少,年龄组成均为衰退型,A项错误;20~24年种群增长率最大,使种群数量维持在该水平可获得最大经济价值,B项错误;第20年时心>1,且维持稳定,种群数量呈“J”型增长,没有K值,C项错误;第20~24年种群数量持续增长,种群年龄组成为增长型,D项正确,

第二节审题指导》本題要求写一封回信,写信属于应用文写作。信件格式已经给出,因此考生只需要根据情境及内容要点正确表达即可。考生可根据要点提示来表达自己对学校严禁带手机进课堂的看法。在写作时,首先表达自己的观点,然后给出理由,最后谈谈自己学校的情况并給予对方安慰。由于谈及的事情均为写信双方的常生活,因此在写作中,主要用第一人称和第二人称,时态以一般现在时或现在完成时为主。【参考范文】Dear PeterI'm sorry to know that you are not allowed to takecellphone into your classroom, but I think it'sfor you to do so. Here's my ideas about itOn the one hand. there are dozens of students inyour class. If someone forgets to turn off his/hercellphone, it may interrupt the class withmessages or calls. On the other hand, if you have acellphone on you when having classes, it's likely thatyou cant concentrate on your classes. for you maywant to look up a word or something else on theInternetIn my school, we are not allowed to takecellphones into our classrooms. At first, we aren'tused to it but now we dont feel it is inconvenientTherefore, I hope you will get used to your schoolrules soon and that you wont be worried about itBest wishesYoursLi hua

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