Vector $\vec{b}$ is obtained by rotating $\vec{a}=\hat{\imath}+\hat{\jmath}$ by $90^{\circ}$ about $\hat{k},$ where $\hat{\imath}, \hat{\jmath}$ and $\hat{k}$ are unit vectors along the $x, y$ and $z$ axes, respectively. $\vec{b}$ is given by
(A) $\hat{\imath}-\hat{\jmath}$
(B) $-\hat{\imath}+\hat{\jmath}$
(C) $\hat{\imath}+\hat{\jmath}$
(D) $-\hat{\imath}-\hat{\jmath}$
Vector $\vec{b}$ is obtained by rotating $\vec{a}=\hat{\imath}+\hat{\jmath}$ by $90^{\circ}$ about $\hat{k},$ where $\hat{\imath}, \hat{\jmath}$ and $\hat{k}$ are unit vectors along the $x, y$ and $z$ axes, respectively. $\vec{b}$ is given by
(A) $\hat{\imath}-\hat{\jmath}$
(B) $-\hat{\imath}+\hat{\jmath}$
(C) $\hat{\imath}+\hat{\jmath}$
(D) $-\hat{\imath}-\hat{\jmath}$