Snapping three days of gains, the Sensex at the Bombay Stock Exchange declined 51 points, or 0.3 percent, to close at 19,939, on profit-booking by investors, today. During the session, the Sensex had again crossed the 20,000 mark. The Sensex had gained 415 points in the previous three trading sessions. The Nifty at the National Stock Exchange fell 19 points, or 0.3 percent, to 6,050. Stock markets in Japan, China, Hong Kong, Singapore and South Korea ended mixed.