The Marshall-Lerner condition applies only if ηx+ηm ＞ 1, in whichηx+ηm is（）
选项：

A:supply price elasticity of domestic import and export commodities
B:demand income elasticity of domestic imports and exports commodities
C:expected Elasticity of demand for domestic imports and exports commodities
D:demand price elasticity of domestic imports and exports commodities