Read these two paragraphs from the PRO article.
Wind power and solar power can also produce energy without releasing carbon. However, they are not constant. To work, the sun has to be out, or the wind has to be blowing.
Nuclear power plants run 24 hours a day, seven days a week. They are the only constant source of power that does not emit greenhouse gases.
How do these paragraphs use a comparison to support a claim?