What is the output of the following program?

#include

using namespace std;

void X$($int A$,$ int &B$)$

$\{$

A $=$ A $+$ B;

B $=$ A $-$ B;

A $=$ A $-$ B;

$\}$

int main$()$

$\{$

int a $=4,$ b $=18$;

X$($a$,$b$)$;

cout $<<$ a $<<","<<$ b;

return $0$;

$\}$

a$.$

$4,4$

b$.$

$18,4$

c$.$

$18,18$

d$.$

$4,18$