题干

下列实验的失败原因可能是因为缺务必要的实验步骤造成的是 (    )

①将乙醇和乙酸、稀硫酸共热制乙酸乙酯

②无水乙醇和浓硫酸共热到140℃制乙烯

③验证RX是氯代烷,把RX与烧碱水溶液混合加热后,将溶液冷却后再加入硝酸银溶液出现褐色沉淀

④做还原性实验时,当加入新制的氢氧化铜悬浊液后,未出现红色沉淀⑤检验淀粉已经水解,将淀粉与少量稀硫酸加热一段时间后,加入银氨溶被水浴加热后未析出银镜

⑤检验淀粉已经水解,将淀粉与少量稀硫酸加热一段时间后,加入银氨溶液水浴加热后未析出银镜

A:①③④⑤   

B:③④⑤   

C:③⑤   

D:全部

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答案(点此获取答案解析)

B

同类题3

I’m an American . Years ago,I traveled to the north of Thailand. I  the heat,the noisy streets,the busy markets and the smell of Thai dish es. I enjoyed my     there very much. However,there is one thing that I wish I hadn’t done.

One day,I set off from my hotel to a temple that    me. It was a very    day. As I walked past some buildings,I glanced between two    buildings. There,an old woman in traditional clothing  on the dusty rubble(碎石).The terrible thought crossed my mind that she might be    .Feeling quite helpless,I  her. Her face was sunkissed. She looked quite beautiful but very ill. I    reached into my pocket to find some   .Then I pushed it into her hand. It was enough for her to   for a few months. At this moment,she opened her large,brown eyes and  the prize I’d given he r. I felt like the “  stranger”.

  a common language,she looked at me with the wisdom and pity that could burn into a soul. She weakly   the money. She stood up and walked away. I was  ,with the money in my hand. It took me at least a kilometer of walking to  I was quite the fool. She didn’t need money.  she needed shade,a bottle of water or just some comfort. She was the one who taught me a good  .

That evening,I sat thinking over my “kindness”. This b eautiful woman made me know that not everyone would  someone else’s money. We should be kind to others,but we should choose the right way.