下列代码中，s1->draw(); 和 s2->draw();输出不同结果的主要原因是（ ）。
class Shape {public: virtual void draw() { cout << "绘制图形" << endl; } virtual ~Shape() {}}; class Circle : public Shape {public: void draw() override { cout << "绘制圆形" << endl; }}; class Rectangle : public Shape {public: void draw() override { cout << "绘制矩形" << endl; }}; int main() { Shape* s1 = new Circle(); Shape* s2 = new Rectangle(); s1->draw(); s2->draw(); delete s1; delete s2; return 0;}