how to determine sizeof class with virtual functions?

this is kind of homework question. For the following code,

#include <iostream>
using namespace std;

class A
{
public:
    virtual void f(){}
};

class B
{
public:
    virtual void f2(){}
};

class C: public A, public B
{
public: 
    virtual void f3(){}
};

class D: public C
{
public:
    virtual void f4(){}
};

int main()
{
    cout<<sizeof(D)<<endl;
}

The output is: 8

Could anyone please explain how it is 8 bytes? If the vtable implementation is compiler dependent, what should I answer for this kind of question in interviews? What about virtual base classes?

EDIT: i am working on a 32-bit platform.