Help with virtual function and virtual inheritance

[shaxquan]

hi, anyone can help me. i have below codes

class A
{
	char k[3];
public:
	virtual void aa() {}
};
class B : public virtual A
{
	char j[3];
public:
	virtual void bb() {}
};

class C : public virtual B
{
	char i[3];
public:
	virtual void cc() {}
};

class D : public B, public C
{
	char l[3];
	virtual void dd() {}
};
int main (int argc, char *argv[]) {
	cout << "A: " << sizeof(A) << endl
		<< "B: " << sizeof(B) << endl
		<< "C: " << sizeof(C) << endl
		<< "D: " << sizeof(D) << endl;
	return 0;
}

the output is
A: 8
B: 16
C: 24
D: 36

I don't why , especially the last one.
I know The A is 8, because there is a pointer point to the virtual function table as well.
In B and C, if there are no virtual, the output would be B: 12, C: 16 and D would be 28.
so can anybody explain to me why this happens, what's the principle.
thanks for your help!