If $I=\left[\begin{array}{ll}1 & 0 \\ 0 & 1\end{array}\right], J=\left[\begin{array}{rr}0 & 1 \\ -1 & 0\end{array}\right]$ and $B=\left[\begin{array}{rr}\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta\end{array}\right]$, then $B$ equals
(a) $/ \cos \theta+J \sin \theta$
(b) $/ \sin \theta+J \cos \theta$
(c) $/ \cos \theta-J \sin \theta$
(d) $-/ \cos \theta+J \sin \theta$
(a) $I \cos \theta+J \sin \theta$
Here,
$I \cos \theta+J \sin \theta$
$=\left[\begin{array}{ll}1 & 0 \\ 0 & 1\end{array}\right] \cos \theta+\left[\begin{array}{cc}0 & 1 \\ -1 & 0\end{array}\right] \sin \theta$
$=\left[\begin{array}{cc}\cos \theta & 0 \\ 0 & \cos \theta\end{array}\right]+\left[\begin{array}{cc}0 & \sin \theta \\ -\sin \theta & 0\end{array}\right]$
$=\left[\begin{array}{cc}\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta\end{array}\right]=B$