This question is based on the following paragraph. the sentences are numbered to you answer the question.

(1) scientists generally agree that earth is getting warmer due to what's called the greenhouse effect. (2) a greenhouse stays warm because sunlight and heat coming through the glass roof are mostly trapped. (3) the heat doesn't radiate back into the atmosphere because it's held in by the glass. (4) something like that happens on earth. (5) the atmosphere acts like a blanket that keeps the sun's heat from leaking away into space.

which sentence is the paragraph's topic sentence?