Investors react in front of screens containing stock information at a brokerage house in Huaibei, Anhui province December 15, 2011. China's main stock index closed down 2.1 percent on Thursday, breaking through a key psychological level, as resource-related stocks took a beating after a broad physical commodities sell-off on lingering risk aversion in global markets. REUTERS/Stringer (CHINA - Tags: BUSINESS) CHINA OUT. NO COMMERCIAL OR EDITORIAL SALES IN CHINA