The BSE benchmark Sensex extended its gains for the sixth day by adding another 104 points in opening trade on Thursday. The 30-share index of the Bombay Stock Exchange, which has gained over 1,145 points in the previous five sessions, moved up further by 104.42 points, or 0.56 per cent, to 18,798.28 in opening trade. In a similar fashion, the broad-based National Stock Exchange Nifty index rose by 28.75 points, or 0.51 per cent, to 5,629.20.