The Sensex at the Bombay Stock Exchange declined 110 points, or 0.6 percent, to close below the 20,000 mark, at 19,895 on Thursday, amid some profit-booking, the monthly expiry in the derivatives segment, and mostly weak global markets.The Nifty at the National Stock Exchange fell 21 points, or 0.4 percent, to 6,035.Stock markets in Japan, China, Hong Kong, South Korea and Singapore also ended largely weak.