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为什么进行直接电位测量时，通过系统的电流必须很小
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Dear Mr. Brown:It was a pleasure to talk with you last Thursday about the executive secretarial position in your company. Your company seems to have an extremely efficient and friendly office staff; I know that I would enjoy the challenge of working in this company.Thank you for giving me so much of your time. I look forward to hearing from you again.Sincerely yoursLi Hua
简述荧光光度分析法的定义及特点。
John F. Kennedy was the youngest American to be elected president of the United States. During the World War Ⅱ John F. Kennedy had an outstanding record in the Navy. After the war Kennedy worked briefly as a newspaperman, then entered politics. He was three times elected to the House of Representatives before becoming a senator from Massachusetts. In 1960 he was elected president in a very close race in which there were charges of irregularities.Kennedy’ s term of office was marked by the achievements in appeals to idealism, a broad participation of people in politics, and improved civil rights. In international affairs, Kennedy saw the failure of the American-backed invasion of Cuban missile crisis which brought Russia and the United States to the brink of war.The Kennedy family has experienced victories and tragedy. One of John Kennedy’ s sisters was mentally backward and an old brother was killed in the war. John Kennedy was assassinated—as was his brother, Robert Kennedy. The family has suffered scandals of sex, alcohol, and drugs in both John’ s generation and in this present generation. Many believe that for the young, wealth and a prominent family too much, too soon, too easy are more of a curse than a help. We learn from the second paragraph that（）
I love traveling by train. Fast expresses, slow local trains which stop at every station, suburban trains taking business men to their offices and home again;I enjoy them all It must be the element of romance that attracts me. There’s no romance in a car on a motor way-a box of metal and rubber(橡胶) on a strip of concrete-or in flying through the air in a pressurized tube from one identical plastic and glass airport to another. But trains are different. You can Walk around, look at the scenery, observe your fellow passengers, In a plane all you can see are the clouds and the backs of other people’ s heads. And then there are the stations.Yes, traveling by train is still an adventure, even in England, as you try to interpret the timetable, persuade the booking office clerk to sell you a ticket and understand the incomprehensible messages coming over the loudspeaker system. Then there’ s that delightful uncertainty as you wonder whether you are on the right train, or the right part of the train. Why does the author dislike traveling by air Because it is（）
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