一道面试题

虽然看过不少类似题，但我还是做错了！谁能详细为我讲解一下啊~

#include <iostream>
using namespace std;

class A
{
public:
    virtual void Fun(int number = 10)
    {
        std::cout << "A::Fun with number " << number<<endl;
    }
};

class B: public A
{
public:
    virtual void Fun(int number = 20)
    {
        std::cout << "B::Fun with number " << number<<endl;
    }
};

int main()
{
    B b;
    A &a = b;
    a.Fun();
 return 0;
}