It is known that some brands of peanut butter contain impurities within an acceptable level. A test conducted on 12 randomly selected jars of a certain brand of peanut butter resulted in the following percentages of impurities:

(a) Construct a 95% jackknife confidence interval for the average percentage of impurities in this brand of peanut butter.
(b) Give an approximate 95% jackknife confidence interval for the population variance.
(c) Interpret your results.

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1.9 2.7 2 2.8 2.3 3.6 1.4 1.8 21 3.2 2.0 1.9