The Bombay Stock Exchange benchmark Sensex today opened lower by nearly 40 points on emergence of profit-selling by investors and others market players.The 30-share index, Sensex, which had gained 145 points in the previous trading session, lost 39.82 points at 16,846 in the first five minutes of trade.The wide-based National Stock Exchange index Nifty declined by 13 points to 5,006.90 points at the same time.