Rising for the fifth straight session, the Sensex at the Bombay Stock Exchange gained 79 points, or 0.4 percent, to 20,608, on monday. The Sensex had gained 633 points in the previous four trading sessions. The Nifty at the National Stock Exchange rose 17 points, or 0.3 percent, to 6,113.Stock markets in China, South Korea and Singapore ended mixed.