[单选] 

31、对下面程序描述正确的是（    ）。

  #include <stdio.h>

  #include <stdlib.h>

   void main()

   {FILE *in,*out;

if((in=fopen("file1.txt","a+"))==NULL)

      {printf("cannot open file1\n");

        exit(0);       }

if((out=fopen("file2.txt","a+"))==NULL)

      {printf("cannot open file2\n");

        exit(0);       }

while(!feof(out))

     fputc(fgetc(out),in);

fclose(in);

fclose(out);   }