Which sentence shows the main idea of the passage?
Electricity fueled by coal is one of the biggest causes of global warming. The coal is burned to heat water and turn it into steam. The steam turns turbines connected to generators that produce electricity. The process gives off huge amounts of carbon dioxide in the atmosphere. In 2011, 1.7 billion tons of carbon dioxide was released into the atmosphere in the United States. Burning coal also gives off a variety of other harmful chemicals like carbon monoxide and nitrogen oxides. These gases, along with carbon dioxide, are the main causes of the greenhouse effect.
