$.$ The type of nuclei that are least stable are those with an $\_\_\_\_\_\_\_\_\_$ number of

neutron and an $\_\_\_\_\_\_\_\_\_\_$ number of protons.

$($a$)$ odd, odd

$($b$)$ no general rule exists

$($c$)$ odd, even

$($d$)$ even, odd

$($e$)$ even, even