Snapping two days of gains, the Sensex at the Bombay Stock Exchange fell 51 points, or 0.2 percent, to close at 21,143, on mild selling by investors with the calendar year drawing to a close. The Nifty at the National Stock Exchange lost 23 points, or 0.4 percent, to 6,291.Stock markets in Japan, Singapore and South Korea gained between 0.1 percent and 0.7 percent.