TORONTO, May 15 (Reuters) - Canadian stocks sank to a seven-month low on Tuesday as resource shares continued their May sell-off after Greece said it would hold new elections, heightening concern about its possible exit from the euro zone and the spillover effect on global markets. The Toronto Stock Exchange's S&P/TSX composite index unofficially closed down 145.40 points, or 1.3 percent, at 11,343.13.