Greece led a global sell-off in May in the face of the eurozone debt crisis and doubts over China's growth. Performance for the month was in stark contrast to April, when ETFs tracking Vietnam, natural gas and home construction were up 9% to 5%. In May, Global X FTSE Greece 20 ETF (GREK) -- the only ETF tracking the beleaguered country -- cratered 33% for the month. By contrast, iShares MSCI EAFE Index (EFA) -- tracking developed foreign markets --