Rising for the third straight session, the Sensex at the Bombay Stock Exchange gained 171 points, or 0.9 percent, to close above the key 20,000 point mark after a gap of five weeks, at 20,060, amid firm global markets, on Tuesday.The Nifty at the National Stock Exchange added 54 points, or 0.9 percent, to 6,001. Stock markets in Japan, China, Hong Kong, South Korea and Singapore rose between 0.2 percent and 1.6 percent.