What will be the output of the following C++ code?

#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

Answer: Option D