有以下程序： #include ＜iostream＞ #include ＜math＞ using namespace std; class point { private:double x;double y; public:point(double a,double b){ x=a; y=b;}friend double distance(point a,point b) ; }; double distance(point a,point b) {return sqrt ((a.x-b.x)* (a.x-b.x)+(a.y-b.y)*(a.y-b.y)); } int main ( ) {point pl(1,2);point p2 (5, 2);cout＜＜distance (pl,p2) ＜＜end1;return 0; } 程序运行后的输出结果是( )。

A．1

B．5

C．4

D．6