Electricity fueled by coal is one of the biggest causes of global warming.
This sentence shows the main idea of the passage. From the answer choices, it is clear that the passage explains how burning coal creates electricity. It includes details such as "coal is burned to heat water and turn it into steam" which then "turns turbines connected to generators." It goes on to explain how it is dangerous to the environment. The passage states that "burning coal gives off a variety of other harmful chemicals...these gases, along with carbon dioxide, are the main causes of the greenhouse effect." All of these details support the main idea that electricity fueled by coal is one of the biggest causes of global warming.
