# General solution of $(2x^3-xy^2)dx+(2y^3-x^2y)dy=0$ is
$(a)\;x^4+x^2y^2-y^4=c \\ (b)\;x^4-x^2y^2+y^4=c \\ (c)\;x^4-x^2y^2-y^4=c \\ (d)\;x^2+x^2y^2+y^4=c$