An advertisement for a popular supermarket chain claims that it has had consistently lower prices than one of its competitors. As part of a survey conducted by an independent price-checking company, the average weekly total, based on the prices of approximately 95 items, is given for this chain and for its competitor recorded during four consecutive weeks in a particular month.

a. Is there a significant difference in the average prices for these two different supermarket chains?
b. What is the approximate p-value for the test conducted in part a?
c. Construct a 99% confidence interval for the difference in the average prices for the two supermarket chains. Interpret this interval.

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Week Advertiser (S) Competitor (S) 254.26 240.62 231.90 234.13 256.03 255.65 255.12 261.18