Falling for the third straight session, the Sensex at the Bombay Stock Exchange dropped 155 points, or 0.9 percent, to close below the 18,000 level, at 17,924 on Friday, on continued profit-booking by investors. The Nifty at the National Stock Exchange declined 54 points, or 1 percent, to 5,429. But stock markets in Japan, China, Hong Kong, Singapore and South Korea rose between 0.1 percent and 1.3 percent, on strong economic data from the US.