Acid rain - characterized as containing harmful levels of sulfur dioxide or nitrogen oxide; acid rain is damaging and potentially deadly to the earth's fragile ecosystems; acidity is measured using the pH scale where 7 is neutral, values greater than 7 are considered alkaline, and values below 5.6 are considered acid precipitation; note - a pH of 2.4 (the acidity of vinegar) has been measured in rainfall in New England.
<em>CIA World Factbook Notes and Definitions
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Acid rain is the rainfall containing elevated levels of sulfuric and nitric acids which turns it acidic in nature. This acidic rain caused by atmospheric pollution is fatal for environmental health as it is harmful to the life of plants, aquatic animals and the infrastructure. Thus, except for this one, all other options are friendly to environmental health. Biomass is the organic fuel used to generate electricity while afforestation would help to decrease the pollution and assist in the purification of nature. Ecosystems comprise the community of living organisms along with their physical environment. Thus, acid rain is the definition that Ted should include as a harmful agent to the environment.
While it is difficult to determine exactly how many Natives lived in North America before Columbus, estimates range from a low of 2.1 million to 7 million people to a high of 18 million.
