TOEFL Reading: ETS-TOEFL阅读机经 - 53ZNUZAQWB7965X8L$

Which of the following helps explain why "the most extensive exploitation of agriculture occurred in river valleys?" A. In river valleys farmers did not have to depend on rain for water. B. The soil in river valleys did not require irrigation. C. Swampy areas in river valleys were easy to drain. D. The expanding populations in river valleys provided large pools of labor.