$(10$ points$)$ Consider the characteristic equations given in each of the following parts and comment on the time response $($is it stable or unstable, is it oscillatory or not$).$

$($a$){}^2+3+2=0$

$($b$){}^2+7+20=0$

$($c$){}^2+25=0$

$($d$){}^2-2+1=0$