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
