单选题 What will be displayed by the following program?#include <iostream>using namespace std;class C {public: string toString( ) { return "C"; }};class B: public C { string toString( ) { return "B"; }};class A: public B { string toString( ) { return "A"; }}; void displayObject(C *p) { cout << p->toString( );} int main( ) { A a; B b; C c; displayObject(&a); displayObject(&b); displayObject(&c); return 0;}