派生类的派生类的构造函数该如何写？

class A
{
public:
        A(int a,int b):num(a),sum(b){}
private:
    int num,sum;
};

class B : public A
{
public:
       B(int a,int b,int c):A(a,b),num(c){}
private:
        int num;
}

class C : public A , public B
{
  public:
        C的构造函数如何书写？