There are two concentric spheres in the figure. The outer sphere is charged to $+16$ nC and has a radius of $2$ cm $,$ the inner sphere is charged to $-5$ nC and has a radius of $1$ cm $.$

$($a$)(1$ point$)$ What is the potential at $r=0?$

$($b$)(1$ point$)$ What is the potential at $r=1cm?$

$($c$)(1$ point$)$ What is the potential at $r=1\mathrm{.}5cm?$

$($d$)(1$ point$)$ What is the potential at $r=2cm?$