Giving up all its initial gains, the Sensex at the Bombay Stock Exchange dropped 90 points, or 0.5 percent, to 16,882, amid weak Asian and European markets, today. The Nifty at the National Stock Exchange fell 31 points, or 0.6 percent, to 5,115.Stock markets in Japan, China, Hong Kong, South Korea, and Singapore lost between 0.5 percent and 1.5 percent, on global growth concerns.