Extending its losses for the fourth straight session, the Sensex at the Bombay Stock Exchange dropped 100 points, or 0.6 percent, to close below the 17,000 level, at 16,988. The market fell on continued concerns that Greece's sovereign debt crisis might spread to European nations like Spain and Portugal, and impact global economic recovery. The Nifty at the National Stock Exchange slipped 34 points, or 0.7 percent, to 5,091.Stock markets in Japan, China, Hong Kong, South Korea and Singapore lost between 0.7 percent and 4 percent.