Posting its second straight day of gains, the Sensex at the Bombay Stock Exchange zoomed 421 points, or 2.7 percent, to 15,939, on Tuesday, on hopes of monetary policy easing, and rising regional markets. The Nifty at the National Stock Exchange rallied 129 points, or 2.8 percent, to 4,765. Stock markets in Hong Kong , South Korea and Singapore rose between 1.6 percent and 2.7 percent.