下列程序的执行结果是______。 #include＜iostream.h＞ class Student { public: Student(int xx){x=xx;} virtual float calcTuition( ); protected: int x; }; float Studertt::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( )＜＜endl; //计算学生s和研究生gs的学费 }