请阅读以下一段程序，并给出答案

#include <iostream>
using namespace std;
class A
{
public:
    A(){ doSth(); }
    virtual void doSth(){cout<<("I am A");}
};
class B:public A
{
public:
    virtual void doSth(){ cout<<("I am B");}
};

int main()
{
 B b;
 return 0;
}

I am A

#include <iostream>
using namespace std;
class A
{
public:
    A(){ doSth(); }
     void doSth(){cout<<("I am A");}
};
class B:public A
{
public:
     void doSth(){ cout<<("I am B");}
};

int main()
{
 B b;
 return 0;
}

I am A

#include <iostream>
using namespace std;
class A
{
public:
    A(){}
    virtual void doSth(){cout<<("I am A");}
};
class B:public A
{
public:
    virtual void doSth(){ cout<<("I am B");}
};

int main()
{
 B b;
 b.doSth();
 return 0;
}

I am B

 

#include <iostream>
using namespace std;
class A
{
public:
    A(){}
     void doSth(){cout<<("I am A");}
};
class B:public A
{
public:
     void doSth(){ cout<<("I am B");}
};

int main()
{
 A a;
 a.doSth();
 return 0;
}

I am A

 

#include <iostream>
using namespace std;
class A
{
public:
    A(){}
     virtual void doSth(){cout<<("I am A");}
};
class B:public A
{
public:
     void doSth(){ cout<<("I am B");}
};

int main()
{
 A a;
 a.doSth();
 return 0;
}

I am A