Marshall-Lerner condition is that the payments deficit will be improved as a result of currency depreciation only if_______ 。( ) A: the sum of elasticity of demand for goods import and that for goods export equals one. B: the sum of elasticity of demand for goods import and that for goods export is less than one. C: the sum of elasticity of demand for goods import and that for goods export is larger than one. D: the sum of elasticity of demand for goods import is greater than that for goods export.