Posting its fifth straight day of gains, the Sensex at the Bombay Stock Exchange rose 55 points, or 0.3 percent, to 18,817, today. The Sensex had climbed over 330 points in the previous four trading sessions. The Nifty at the National Stock Exchange gained 20 points, or 0.4 percent, to 5,724.But stock markets in Japan, China, Hong Kong, and Singapore dropped about 0.4 percent, each, on investor caution ahead of the US Presidential elections.