题干

甲校有3600名学生。乙校有5400名学生,丙校有1800名学生,为统计三校学生身高方面的情况,计划采用分层抽样法,抽取一个样本容量为90人的样本,则应在这三校分别抽取学生(  )

A:30人,30人,30人

B:30人,45人,15人

C:20人,30人,10人

D:30人,50人,10人

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答案(点此获取答案解析)

B

同类题3

碳、氮、硫是中学化学重要的非金属元素,在工农业生产中有广泛的应用.

(1)用于发射“天宫一号”的长征二号火箭的燃料是液态偏二甲肼(CH3)2N﹣NH2,氧化剂是液态四氧化二氮.二者在反应过程中放出大量能量,同时生成无毒、无污染的气体.已知室温下,1g燃料完全燃烧释放出的能量为42.5kJ,请写出该反应的热化学方程式____ 

(2)298K时,在2L的密闭容器中,发生可逆反应2NO2(g)⇌N2O4(g)△H=﹣a kJ•mol﹣1 (a>0).N2O4的物质的量浓度随时间变化如图1.达平衡时,N2O4的浓度为NO2的2倍,回答下列问题.

①298k时,该反应的平衡常数为____ 

②在温度为T1、T2时,平衡体系中NO2的体积分数随压强变化曲线如图2所示.下列说法正确的是____ 

a.A、C两点的反应速率:A>C         

b.B、C两点的气体的平均相对分子质量:B<C

c.A、C两点气体的颜色:A深,C浅     

d.由状态B到状态A,可以用加热的方法

③若反应在398K进行,某时刻测得n(NO2)=0.6mol n(N2O4)=1.2mol,则此时V(正)____ V(逆)(填“>”、“<”或“=”).

(3)NH4HSO4在分析试剂、医药、电子工业中用途广泛.现向100mL 0.1mol•L﹣1NH4HSO4溶液中滴加0.1mol•L﹣1NaOH溶液,得到的溶液pH与NaOH溶液体积的关系曲线如图3所示.试分析图中a、b、c、d、e五个点,

①水的电离程度最大的是____ 

②其溶液中c(OH﹣)的数值最接近NH3•H2O的电离常数K数值的是____ 

③在c点,溶液中各离子浓度由大到小的排列顺序是____ 

同类题4

    When people introduce themselves,  I always hear them say "I'm just ahousewife".  I don't understand whytheir introduction is always filled with regret and self - pity.  Is it, really a crime to be a housewife? Isit really embarrassing when you choose not to work outside and instead chooseto work in  your home?

    How could you be "just ahousewife" when the basic social unit of society depends on you and yourservice? A housewife works for the longest hour, supporting your husband, childstudies, caring for eld-ers, managing finances, paying bills, grocery shopping,cooking according to individual taste, cleaning, etc. A housewife could beanyone: daughter, wife, mother, teacher, financial advisor, fashion designerand nurse, gardener, driver, cook, and so on. Don't think you are not being paid for it.  You get the re-ward that no money canoffer-the love and affection of your family, their trust in you, ahappy home and your own satisfaction.

    It is you who choose to be in thisoccupation, willingly or for any other reason. After a busy day, all wait to return to the comfortable home set by you.They know you will be there to absorb all their stress and tiredness and relaxthem. Would the life have been the same had you been working outside to earnsome money? Would you along with your family have been able to buy all thecontent in life that you are enjoying now with that money?

    Respect yourself for what you are. Introduce yourself to the world with the same dignity. The world understandsthe importance of your job; it's time to make them accept it. Introduceyourself with pride and with twinkle in your eyes "I am a housewife".Kick that "just" out.