这个是什么错误？

我按c primer上说的写了个计算中奖率的程序，可是编译时出现了问题，我的英文水平一般，也看不懂什么意思，大家帮我看看错哪了，程序如下：
# include <iostream>
long double pro(unsigned numbers,unsigned picks);
int main()
{ using namespace std;
  double a,b;
  cout<<"请输入总数字和可选数字个数\n";
  while((cin>>a>>b)&&b<=a)
  {cout<<"你的中奖率为:";
    cout<<pro(a,b);
    cout<<"输入Q以结束输入";
   }
   cout<<"bye\n";
   system("pause");
   return 0;
}
 long double pro(unsigned numbers,unsigned picks)
 { long double c=1.0;
   long double n;
   unsigned d;
   for(n=numbers,d=picks;d>0;n--,d--)
    c=c*n/d;
    return c;
}
   错误提示如下：
   9 C:\Documents and Settings\cface\桌面\新建文件夹\Untitled1.cpp [Warning] passing `double' for converting 1 of `long double pro(unsigned int, unsigned int)'
   9 C:\Documents and Settings\cface\桌面\新建文件夹\Untitled1.cpp [Warning] passing `double' for converting 2 of `long double pro(unsigned int, unsigned int)'