[单项选择]请找出下列程序中错误之处 ______。 #include<iostream.h> class A private: int x1; protected: int x2; public: int x3; ; class B:public A private: int y1; protected: int y2; public: int y3; void disp( )cout<<x1<<y1<<end1: //A void set(int i) x2=i; //B ; void main( ) B bb; bb.x3=10; //C bb.y3=10; //D