6. In the Cartesian coordinate system, the curve represented by the equation $\frac{|x+y|}{2 a}+\frac{|x-y|}{2 b}=1(a, b$ are two unequal positive numbers) is
A. Triangle
B. Square
C. Non-square rectangle
D. Non-square rhombus
Solve:
Let $x=y \Rightarrow A(a, a), C(-a,-a)$, let $x=-y \Rightarrow B(-b, b), D(b,-b)$. Therefore, quadrilateral $A B C D$ is a rhombus that is not a square, so the answer is $D$.

