下列关于功、功率、机械效率的说法中，正确的是（   ）

中国古代历史上屡次出现南北分裂或对峙，最终多以北方政权统一南方政权为结局。以下哪次统一却改变了这一“定势”？

赫鲁晓夫在上台之初说：“我们共产党员好像是布道的神父，许诺说天上有个天国，可眼下却没有土豆吃。我们又不是神父，我们是共产党人，我们应当在地面上给予这种幸福。”对这段话理解正确的是（    ）

   (One day Alice runs after a strange rabbit and comes into a deep hole. She becomes smaller when she wakes up the next day. She finds herself in a wonderland with many amazing things inside.)
   The Caterpillar(毛毛虫) and Alice looked at each other for some time in silence. At last the Caterpillar took the pipe out of its mouth, and said to her in a tired, sleepy voice.
   “Who are YOU?” said the Caterpillar.
   This opening for a talk couldn’t make Alice at ease. Alice replied, rather shyly, “I----I hardly know, sir, just at present ----at least I know who I was when I got up this morning, but I think I must have been changed several times since then.”
   “What do you mean by that?” said the Caterpillar, “Explain yourself!”
   “I can’t explain MYSELF. I’m afraid, sir.” said Alice, “because I’m not myself, you see.”
   “I don’t see.” said the Caterpillar.
   “I’m afraid I can’t put it more clearly.” Alice replied very politely, “for I can’t understand it myself. And being so many different sizes in a day is very puzzling.”
   “It isn’t.” said the Caterpillar.
   “Well, perhaps you haven’t found it so yet.” said Alice. “But when you have to turn into a chrysalis(蛹，茧)----you will some day, you know ----and then after that into a butterfly, I think you’ll feel it a little stranger, won’t you?”
   “Not a bit.” said the Caterpillar.
   “Well, perhaps your feelings may be different.” said Alice. “All I know is, it would make ME feel very strange.”
   “You!” said the Caterpillar scornfully. “Who are YOU?” The question brought them back again to the beginning of the talk. Alice felt a little angry because the Caterpillar was making such VERY short sentences. She drew herself up and said very seriously, “I think you ought to tell me who YOU are first.”
   “Why?” said the Caterpillar.
   Here was another puzzling question. As Alice could not think out any good reason, and as the Caterpillar seemed to be very unpleasant, she turned away.
   “Come back!” the Caterpillar called after her. “I’ve something important to tell you!”
   This certainly gave Alice some hope. Alice turned and came back again.

合物Z由如下反应得到：C₄H₉Br $\underset{NaOH}{\overset{醇，△}{\to }}$ Y $\underset{CC{l}_4}{\overset{B{r}_2}{\to }}$ Z，Z的结构简式不可能是（   ）