The BSE Sensex extended its gains by adding 90 points in opening trade on Monday to 18905 points on continued buying by foreign funds and retail investors.Similarly, the wide-based National Stock Exchange Nifty index moved up by 22.70 points, or 0.40 per cent, to 5,676.Brokers said sustained buying by funds and retail investors in stocks with strong fundamentals amid expectations of better fourth quarter earnings mainly influenced the trading sentiment.