You enjoy learning about history by studying gravestones throughout North America. You notice that gravestones from 1900 and earlier usually host many types of lichens. But in one cemetery, lichens are entirely absent, even from old gravestones. Given what is known about lichens, the cemetery without lichens probably

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Lichens are known to be sensitive to air pollution. The absence of lichens on the gravestones in this cemetery suggests that the air quality in the area may have been poor during the time period in which the gravestones were first erected.

You Enjoy Learning About History by Studying Gravestones Throughout North