下列程序的执行结果是 【12】 。#include＜iostream. h＞class Student{public: Student(int xx){x=xx;} virtual float calcFuition( );protected: int x;}float Student:: calcTuition(){ return float(x* x)class GraduateStudent:public Student{public: GraduateStudent(int xx) :Student(xx) {} float calcTuition( )};float GraduateStudent:: calcTuition( ){ return float(x * 2);}void main( ){ Student s(20); GraduateStudent gs(30); cout＜＜s.calcTuition()＜＜" "＜＜gs. calcTuition()＝＝end1; //计算学生s和研究生gs的学费}