Falling for the third straight session, the Sensex at the Bombay Stock Exchange dropped 64 points, or 0.3 percent, to close below the 20,000 level, at 19,941, on the expiry of the monthly derivative contracts, and after choppy trade, on Thursday. The Nifty at the National Stock Exchange lost 25 points, or 0.4 percent, to 5,988.