Posting its biggest single-day gain in almost four months, the Sensex at the Bombay Stock Exchange shot up a hefty 513 points, or 2.9 percent, to close above the key 18,000 level, at 18,241, today. The market was lifted by a sharp fall in crude oil prices, and by rising Asian and European markets. The Nifty at the National Stock Exchange surged 151 points, or 2.8 percent, to 5,471.Stock markets in Japan, China, Hong Kong, South Korea and Singapore rose between 0.7% and 2.2%, as European leaders voiced support for Greece, and as a slump in crude oil prices eased concern that global economic recovery will falter.