Does the quality of cell service vary between providers? The file CellService provides the cellphone rating of AT&T and Verizon Wireless in 23 U. S. cities. (Data extracted from " Smart-Phone Shopping Guide," Consumer Reports, January 2014, pp. 30-31.)

a. At the 0.05 level of significance, is there evidence of a difference in the mean cell service rating between AT&T and Verizon Wireless?

b. What assumption about the cellphone rating is necessary to perform this test?

c. Use a graphical method to evaluate the assumption made in (a).

d. Construct and interpret a 95% confidence interval estimate of the difference in the mean cell service rating between AT&T and Verizon Wireless.