Falling for the fifth straight session, the Sensex at the Bombay Stock Exchange dropped 175 points, or 0.9 percent, to close at a nearly four-week low of 20,491,on Monday. The Sensex fell on rupee weakness, amid concerns that the US Federal Reserve will soon start winding-down its monetary stimulus. The Sensex has now lost 748 points in five trading sessions. The Nifty at the National Stock Exchange fell 62 points, or 1 percent, to 6,079.But stock markets in Japan, China, Hong Kong and Singapore gained between 0.2 percent and 1.4 percent.