Snapping two days of gains, the Sensex at the Bombay Stock Exchange dropped 115 points, or 0.6 percent, to 18,494, on profit-booking by investors, amid weak global markets on Thursday.The Nifty at the National Stock Exchange declined 42 points, or 0.7 percent, to 5,550.Stock markets in Japan, China, Singapore, Hong Kong, and South Korea fell between 0.4 percent and 1.7 percent.