Snapping three days of gains, the Sensex at the Bombay Stock Exchange dropped 43 points, or 0.2 percent, to close at 20,855, on profit-booking by investors, amid falling European markets, today. The Sensex had rallied 478 points in the previous three trading sessions. The Nifty at the National Stock Exchange fell 16 points, or 0.3 percent, to 6,202.Stock markets in Japan, China, Hong Kong, South Korea and Singapore again ended mixed.