The graph of $x^{2}-y^{2}=0$ is
(A) a straight line
(B) a parabola
(C) a circle
(D) a single point
(E) two straight lines
If $x^{2}-y^{2}=0$, then $(x-y)(x+y)=0$, so $y=x$ or $y=-x$. These are the equations of two straight lines (each of which passes through $(0,0)$ ).

Answer: (E)