March 17 (Reuters) - Canada’s main stock index opened lower on Wednesday, as investors remained on the sidelines ahead of the U.S. Federal Reserve’s policy decision, while data showed domestic inflation numbers edged higher in February. * At 9:30 a.m. ET (13:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 33.2 points, or 0.18%, at 18,840.81.