Scientists generally agree that Earth is getting warmer due to what's called the greenhouse effect. A greenhouse stays warm because sunlight and heat coming through the glass roof are mostly trapped. The heat doesn't radiate back into the atmosphere because it's held in by the glass. Something like that happens on Earth. The atmosphere acts like a blanket that keeps the Sun's heat from leaking away into space. The paragraph's topic sentence is: Sentence 1.

A muckraker is a writer who exposed abuses of businesses and government.

The statement within a paragraph that provides the central idea is called a topic sentence.

Proteins builds and maintains cells and tissues.