China’s major stock exchanges tanked on the first trading day of the year, triggering a “circuit-breaker” that suspended equities trade nation-wide for the first time and put months of regulatory work to restore market stability at risk. The selloff saw the CSI300 index of the largest listed companies in Shanghai and Shenzhen lose 7.0 percent before trading was suspended, its worst single-day performance since late August 2015, the depth of a summer stock market rout.