编写程序: 从键盘输入正整数n，计算:s = 1-1/2+1/3-1/4+1/5-1/6+1/7-...+1/n。
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绿色许可证输入窗口弹出后()秒内未做任何操作,窗口自动关闭。
A. 60
B. 20
C. 45
D. 15
Which of the following experiments uses the self-indicator method?
A. Standardization KMnO4 solution
B. Standardization NaOH solution
C. Standardization Na2S2O3 solution
Determination of the content of ammonium chloride
When determine the concentration of H2O2 with KMnO4 solution, which can be used to control acidity? （ ）。
A. HNO3
B. HCl
C. HAc
D. H2SO4
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