The statement, ''Baltimore has taken a zero-tolerance approach to polluters, so its citizens are much less exposed to toxic chemicals than the citizens of Phoenix'', perfectly explains the difference between the two cities in the study of urban ecology. Therefore, the option A holds true.
A study of urban ecology can be referred to or considered as the study that takes the factors such as the ecological forces of urban regions and areas into consideration for the purpose of outcomes of the study. The urban ecology of the cities, Baltimore and Phoenix, are completely in contrast to each other with respect to their exposure to toxic chemicals.
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