Snapping three days of gains, the Sensex at the Bombay Stock Exchange dropped 120 points, or 0.6 percent, to close below the 20,000 mark, at 19,982, on profit-booking by investors, amid weak European markets, today. The Sensex had gained over 280 points in the last three trading sessions. The Nifty at the National Stock Exchange, which had crossed the 6,100 mark for the first time in two years, during the session, finally ended the day with a loss of 34 points, or 0.6 percent, at 6,049.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.