单选题 A nursery sells trees of different types and heights. Suppose that 75 pine trees are sold for planting at City Hall. These 75 trees average 60 inches in height with a standard deviation of 16 inches. Using this information, construct a 95 percent confidence interval for the mean height of all trees in the nursery:

A. 60±1.96(16).
B. 0.8±1.96(16).
C. 60±1.96(1.85).

【正确答案】 C

【答案解析】Because we know the population standard deviation, we use the z - statistic. A 95 percent confidence level is constructed by taking the population mean and adding and subtracting the product of the z - statistic reliability ([*]) factor times the known standard deviation of the population divided by the square root of the sample size: [*] (1.96)×(16/8.6603)=60±(1.96)×(1.85).