Which line in the following program contains function header$$for the showDub function?

$1$#include

$2$using namespace std;

$3$

$4$void showDub$($int$)$;

$5$

$6$int main$()$

$7\{$

$8$int x $=2$;

$9$

$10$showDub$($x$)$;

$11$cout $<<$ x $<<$ endl;

$12$return $0$;

$13\}$

$14$

$15$void showDub$($int num$)$

$16\{$

$17$cout $<<($num $*2)<<$ endl;

$18$return;

$19\}$