C++ Friend Question:

What is the output of this program?

#include <iostream>
using namespace std;
class Box
{
double width;
public:
friend void printWidth( Box box );
void setWidth( double wid );
};
void Box::setWidth( double wid )
{
width = wid;
}
void printWidth( Box box )
{
box.width = box.width * 2;
cout << "Width of box : " << box.width << endl;
}
int main( )
{
Box box;
box.setWidth(10.0);
printWidth( box );
return 0;
}

a) 40
b) 5
c) 10
d) 20