The BSE benchmark Sensex today fell by almost 75 points in opening trade due to increased selling by participants. The 30-share index, which had lost over 129 points in the previous session, fell by 0.39 per cent to 18,600, led by stocks of auto and banking sectors. Similarly, the wide-based National Stock Exchange Nifty Index shed 25 points, or 0.44 per cent, to 5,451.Brokers said, participants adopted a cautious approach and trimmed their positions ahead of wholesale inflation data for September, to be released later in the day and Q2 earnings by Reliance Industries, which has significant influence over 30-share Sensex. A weak trend in the Asian region also had its impact on the trading sentiments, they added.