有如下程序：#include ＜iostream＞using namespace std;class point{ int x, y;public: point( int vx, int vy ) {X=vx;y=vy; } point() {x=0;y=0; } point operator+ ( point p1 ) {point p;int px = x + p1.x;int py = y+ p1.y;return point( px, py ); } point operator-( point p1 {point p;int px = x - p1.x;int py = y - p1.y;return point( px, py ); } void print() { cout＜＜x＜＜","＜＜y＜＜end1; }};int main(){ point p1( 10, 10 ), p2( 20, 20 ); p1 = p1 + p2; p1.print(); return ();}执行后的输出结果是( )。

A．10, 10

B．20, 20

C．10, 20

D．30, 30

旋涡泵流量用（）调节，离心泵流量用（）调节。