运行下列程序时的输出结果是( )。#includeusing namespace std;class A{public: virtual void fun(){cout<<'A';}};class B:public A{public: void fun(){cout<<'B';}};void f1(A t){t.fun();}void f2(A &t){t.fun();}void f3(A *t){t->fun();}int main(void ){ B b1; f1(b1);f2(b1);f3(&b1); return 0;}

AAA
B. ABB
C. AAB
D. BBB