Rising for the third straight session, the Sensex at the Bombay Stock Exchange gained 172 points, or 0.9 percent, to close at 18,412 on Monday. Public sector oil marketing companies surged, after the government hiked prices of petro-products. The Nifty at the National Stock Exchange rose 55 points, or 1 percent, to 5,527. But stock markets in Japan, Hong Kong, South Korea and Singapore lost between 0.5 percent and 1 percent.