12. A pollution-control inspector suspected that a riverside community was releasing semi- treated sewage into a river and this, as a consequence, was changing the level of dissolved oxygen of the river. To check this, he drew 15 randomly selected specimens of river water at a location above the town and another 15 specimens below. The dissolved oxygen readings, in parts per million, are given in the accompanying table. Above town 5.2 4.8 5.1 5.0 4.9 4.8 5.0 4.7 4.7 5.0 47 6.1 5.0 4 9 4.9 Below town 4.2 4.4 4.7 4.9 4.6 4.8 4.9 4.6 5.1 4.3 5.5 4.7 4.9 4.8 | 4.7 a. Do the data provide sufficient evidence to indicate a difference in mean oxygen content between locations above and below the town? Use o = .05. b. Do the required conditions to use the test in (e) appear to be valid for this study? Justify your answer c. How large is the difference between the mean oxygen content above and below the town