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

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    Speaking in public is most people's least favorite thing. The reason is that we are all afraid of making fools of ourselves.____

    But stop biting your finger-nails. Public speaking is easy. It's just plain talking, and you talk all the time. Although I'm basically shy (honest!), I've been making speeches and talking on radio and television for more than 30 years, and I can tell you that public speaking is not a “gift” like musical talent.____ Here are some of the lessons I have learned:

    Your audience is going to come away with one or two of your main ideas. One or two. Not ten or 20.____ And if you don't have a clear idea of what you want to say, there's no way your audience will.____— how you are going to open, what major points you want to make and how you're going to close.

    When I do a radio or TV piece, I often write the last sentence first.____ A strong close is critical: the last thing you say is what your audience will most likely remember.

    The standard length of a vaudeville (杂耍) act is usually 12 minutes. If all those performers singing and dancing their hearts out couldn't go on longer without boring the audience, what makes you think you can?

A. Some people are born to be good at making speeches.

B. Anybody who can talk can speak in public.

C. Try to relax yourself before making the speech.

D. The more important the speech is, the more frightened we become.

E. If you can't express in a sentence or two what you intend to get across, then your speech is not focused well enough.

F. When you know where you're headed, you can choose any route to get there.

G. No matter how long or short your speech is, you've got to get your ducks in a row.