14. Define $a * b=a^{2} b-a b^{2}$. Then the set of points
$$
\{(x, y) \mid x * y=y * x\}
$$

represents $(\quad)$.
(A) a finite set of points
(B) a line
(C) two parallel lines
(D) two intersecting lines
(E) three lines
14. E.
$$
\begin{array}{l}
\{(x, y) \mid x * y=y * x\} \\
=\left\{(x, y) \mid x^{2} y-y^{2} x=y^{2} x-x^{2} y\right\} \\
=\{(x, y) \mid x y(x-y)=0\} \\
=\{(x, y) \mid x=0 \text { or } y=0 \text { or } x=y\} .
\end{array}
$$