派生类的方法疑问

#include <iostream.h>
class CRoot{
  public:
    int small;
    CRoot()
    {small= 2; }
    CRoot(int n)
    { small = n;}
    void showsmall()
    { cout<<"small="<<small<<endl; }
};
class CDer1:public CRoot
{
  public:
      CDer1(int m):CRoot(m)      { }
};
class CDer2:public CRoot
{
    public:
       int small;
       CDer2(int n=0) {small=n; }
};
void main()
{     CRoot A;
      CDer1 B(3);
      CDer2 C;
      A.showsmall(); B.showsmall(); C.showsmall();
 }
运行结果：
 
为什么是small=2
        small=3
        small=2