TOEFL Reading: ETS-TOEFL阅读机经 - C624T1353863U32BL$

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Researchers have tried to establish why an area as dry as Chaco was the site of such large construction efforts. B. Researchers have tried to establish whether the concentration of massive construction projects in a brief period of time made Chaco the dry area that it is today. C. Researchers have established that Chaco's brief growing season required a concentration of labor to produce large quantities of food in a short period of time. D. Researchers have established that the hot, dry climate of Chaco forced workers to complete construction on large buildings in short periods of time.