TOEFL Reading: ETS-TOEFL阅读机经 - 3PJ5S6662O23TC83L$

Why does the author mention "a period of drought?" A. To help explain why the richest soils in the Near East were located in the deltas at the mouths of the Tigris and Euphrates Rivers B. To suggest a reason for undertaking the massive effort to make the deltas of the Tigris and Euphrates Rivers farmable C. To identify a condition that often affected agricultural production in Mesopotamia D. To support the idea that mobilizing large pools of labor after 4000 B.C.E. required significant administrative organization