Falling for the fifth straight session, the Sensex at the Bombay Stock Exchange declined 91 points, or 0.5 percent, to close at a nearly four-month low of 18,793, on fag-end selling, amid falling European markets, today. The Sensex has now lost nearly 780 points in the last five trading sessions. The Nifty at the National Stock Exchange fell 36 points, or 0.6 percent, to 5,659.Stock markets in Japan, China, Hong Kong, Singapore and South Korea ended mixed.