April 21 (Reuters) - Canada’s main stock index opened flat on Wednesday as a drop in oil prices offset the boost from positive corporate earnings. * At 9:31 a.m. ET (13:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 5.73 points, or 0.03%, at 19,035.05. (Reporting by Shashank Nayar in Bengaluru; Editing by Aditya Soni)