While I was walking to the story because an independent clause mean it can stand on its own as a sentence
The correct answer is acid rain.
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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<em>CIA World Factbook</em>
Catching a bus can be the cheapest way of traveling between cities. It
is often the only way of getting to smaller towns if you are not
driving a car. Remember that bus travel can be very long due to the
great distances involved. Most buses have air conditioning and a
washroom on board. The largest network of bus routes is run by a
company called Greyhound and its affiliates.
