The Sensex at the Bombay Stock Exchange opened 57 points or 0.3 percent lower at 18,231 Tuesday morning on profit-booking by investors and amid weak Asian markets. Falling further, the Sensex stood 98 points or 0.5 percent in the red at 18,189 a shortwhile ago.Other Asian markets in Japan, Singapore, Taiwan, Indonesia, Hong Kong and South Korea were down by between 0.2 percent and 1.2 percent on signs that China's economy continues to slow and ahead of the US central bank's meeting that may bring new measures to prop up sputtering US growth. China's key stock index slid 2.3%.