According to paragraph 2, which of the following was true of the soil in lower Mesopotamia? A. It was relatively unfertile until humans added other, richer types of soil to it. B. It reached full productivity only when elaborate drainage and irrigation systems were added. C. Its most valuable nutrients were washed away by the spring floods of the Tigris and Euphrates rivers. D. Its silt was removed by a large, well-disciplined workforce.