Snapping two days of gains, the Sensex at the Bombay Stock Exchange declined 68 points, or 0.3 percent, to close at 21,033, today, as investors started unwinding their pending positions ahead of the monthly settlement in the derivatives segment. The Sensex had rallied over 390 points in the previous two trading sessions. The Nifty at the National Stock Exchange fell 16 points, or 0.3 percent, to 6,268.Stock markets in Japan, China, and South Korea gained about 0.2 percent, each.