$.$ In the following code, where are variables a$,$ b$,$ c$,$ and d stored?

#include

int a $=10$;

int Add$($int x$,$ int y$)\{$

int b;

b $=$ x $+$ y;

return b;

$\}$

int main$()\{$

int c;

c $=20$;

int d;

d $=$ Add$($a$,$c$)$;

$\}$

$($a$)$ array $($b$)$ stack $($c$)$ list $($d$)$ heap

printf$("\%$d

$",$ d$)$;

return $0$;