To every linear transformation $T$ from ${\mathbb R}^2$ to ${\mathbb R}^2$, there is an associated $2 \times 2$ matrix. Match the following linear transformations with their associated matrix.

 1. Clockwise rotation by $\pi/2$ radians
 2. Counter-clockwise rotation by $\pi/2$ radians
 3. Reflection about the y-axis
 4. Reflection about the line y=x
 5. The projection onto the x-axis given by T(x,y)=(x,0)
 6. Reflection about the $x$-axis

A. $\begin{pmatrix}0&1\\ -1&0 \end{pmatrix}$
B. $\begin{pmatrix}-1&0\\ 0&1 \end{pmatrix}$
C. $\begin{pmatrix}0&-1\\ 1&0 \end{pmatrix}$
D. $\begin{pmatrix}1&0\\ 0&-1 \end{pmatrix}$
E. $\begin{pmatrix}1&0\\ 0&0 \end{pmatrix}$
F. $\begin{pmatrix}0&1\\ 1&0 \end{pmatrix}$
G. None of the above