Dec 7 (Reuters) - Canada’s main stock index opened lower on Monday, dragged down by weakness in energy stocks, while rising tensions between the United States and China sapped appetite for risky assets. * At 09:31 a.m. ET (14:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 38.4 points, or 0.22%, at 17,482.57.