单选题 阅读下列程序: #include<iostream> using namespace std; class A{ public: A() {cout << 'A';} ~A () {cout << 'C';} }; class B: public A { public: B(){ cout << 'G';} ~B () {cout << 'T';} }; int main(){ B obj; return 0; } 程序的运行结果为()