$2\mathrm{.}21$ The direction of vector $A$ is radially outward from the origin, with $|A|=k{r}^n$ where $r^2=x^2+y^2+z^2$ and $k$ is a constant. The value of $n$ for which grad$*A=0$ is

$($a$)-2$

$($b$)2$

$($c$)1$

$($d$)0$

$[2012$: $2$ M$]$