Using C$++$ int mystery$($int first, int last$)$

$\{$

if $($first $>$ last$)$

return $0$;

else if $($first $==$ last$)$

return first;

else

return first $+$ mystery$($first $+1,$ last $-1)$;

$\}$

$$

What is the output of the following statement?

$$

cout $<<$ mystery$(6,10)<<$ endl;

Group of answer choices

$13$

$21$

$40$

$42$