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　　2. A sample has the following characteristics
　    The mean of the sample is 3.5%.
　　Standard deviation is 2.5%.
　　900 observations in the sample.
　　Which is the standard error of the mean estimate?
　　A. 0.117%
　　B. 0.083%
　　C. 0.053%
　　D. 0.075%
　　答案: B