甲、乙为短周期同一主族元素组成的单质，丙、丁为氧化物，它们存在如下转化关系，则满足条件的甲和乙分别为（   ）
  

    It's necessary to discover what you are good at or not and what you like or dislike.
    Many students would be unhappy if they f____  in a very difficult physics course, but in fact they have known a lot about t____  They realize that they should not become engineers or p____ scientists in the future. So falling can help a student to live a happier life if he or she draws something out of it. They may then decide on their aim and c____  the kind of work that they would be fit for and they would like to do.
    You can't be sure whether you like something u____  you have tried it. If you make a d____  that you would like to play the violin, you need to take more music classes b____ you know if you are really interested in it.
    It is not e____  to hope to be a violinist. You also have to stand the hard practice before you become one. If you would enjoy being a great violinist, but hate the hard and b____  work, just give it up.
    It's a good plan to try as many ideas as possible when you are still young. And then you will be able to find o____ what your true interests are.

将各种血细胞名称与其形态结构特点以及主要功能用线连起来。

①红细胞____       A.体积最小，形状不规则，无细胞核            a.吞噬病菌，  防御和保护

②白细胞____       B.有细胞核，体积较大                                 b.止血、凝血

③血小板____       C.两面凹的圆饼状，成熟的细胞无细胞核      c.运输氧和部分二氧化碳，富含血红蛋白

NaHSO₄在水溶液中能够电离出H^＋、Na^＋和SO₄^2－。下列分类中错误的是（）