Which statement BEST states the main idea of the passage?

The last answer best describes the main idea of the passage. You can eliminate (Early civilizations developed along rivers, which served as a source of irrigation water and a highway for trade) and (The rise of civilization about 5,000 years ago is a relatively recent event in the time span of human existence) because these introduce new ideas (the time span of human existence and trade) not covered in the passage. Answer (Early civilizations developed in the valley of the Tigris and Euphrates and in the valleys of the Nile, Indus, and Yellow Rivers) is true but simply restates examples from the last paragraph and misses the main point—the relationship between irrigation, population density, and civilization.