宏定义中printf中的‘#’

先看一例：
#include<iostream>
#include<cstdio>
using namespace std;
#define print(x) printf("the No. "#x", is ")
int main()
{
  print(10);//输出：the No. 10, is
  print("abc");//输出：the No. "abc", is
  print(abc);//输出：the No. abc, is
  int y=10;
  printf("#y");//输出：#y
  cout<<endl;
  return 0;
}
请问printf中的#在宏定义中起什么作用啊？