TOEFL Reading: ETS-TOEFL阅读机经 - TZWU1717KC9YHM87L$

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. Sumeriancities developed a trade in copper and stone that extended as far as Oman in the Persian Gulf and the mountains of Anatolia and the Caucasus. B. Sumerian cities competed with traders elsewhere to produce copper that was more economical and useful than stone. C. In Sumerian cities, imported copper often proved more economical and useful than imported stone. D. The copper and stone imported from Oman, Anatolia, and the Caucasus proved more economical and effective than the copper and stone found in Sumerian cities.