The Sensex at the Bombay Stock Exchange on Wednesday crossed the 18,000-level after more than two years, but then closed below this psychological mark, at 17,970, with a small gain of 29 points, or 0.2 percent. The Nifty at the National Stock Exchange rose 9 points, or 0.2 percent, to 5,375.Stock markets in Japan, Hong Kong, Singapore and South Korea mostly ended with modest gains.