Falling for the second straight session, the Sensex at the Bombay Stock Exchange dropped 84 points, or 0.4 percent, to close at 21,171, today, on investor caution ahead of key economic data, and amid weak regional markets. The Nifty at the National Stock Exchange declined 25 points, or 0.4 percent, to 6,308.Stock markets in Japan, China, Hong Kong, South Korea and Singapore declined between 0.6 percent and 1.7 percent.