16. Given the coordinates of the three vertices of $\triangle A B C$ are $A(0,2), B(-3,2), C(-3,0)$, construct $\triangle A^{\prime} B^{\prime} C^{\prime}$ which is the reflection of $\triangle A B C$ over the $x$-axis, and then rotate $\triangle A^{\prime} B^{\prime} C^{\prime}$ counterclockwise by $90^{\circ}$ around the origin $O$ to obtain $\triangle A^{\prime \prime} B^{\prime \prime} C^{\prime \prime}$. Which of the following statements about $\triangle A B C$ and $\triangle A^{\prime \prime} B^{\prime \prime} C^{\prime \prime}$ is true?
(A) Rotating $\triangle A^{\prime \prime} B^{\prime \prime} C^{\prime \prime}$ counterclockwise by $90^{\circ}$ around the origin results in $\triangle A B C$
(B) Rotating $\triangle A^{\prime \prime} B^{\prime \prime} C^{\prime \prime}$ clockwise by $90^{\circ}$ around the origin results in $\triangle A B C$
(C) The two triangles are symmetric with respect to the $x$-axis
(D) The two triangles are symmetric with respect to the line $y=x$
(E) The two triangles are symmetric with respect to the $y$-axis
16. D.

Consider any point $P(x, y)$ on the coordinate plane. The point symmetric to $P$ with respect to the $x$-axis is $P^{\prime}(x,-y)$. Then, rotating point $P^{\prime}$ counterclockwise by $90^{\circ}$ around the origin $O$, we get $P^{\prime \prime}(y, x)$. Therefore, $P$ and $P^{\prime \prime}$ are symmetric with respect to the line $y=x$.