对下面程序描述正确的是()。#includestdio.h#includestdlib.hvoidmain(){FILE*in,*out;if((in=fopen("file1.txt","a+"))==NULL){printf("cannotopenfile1\n");exit(0);}if((out=fopen("file2.txt","a+"))==NULL){printf("cannotopenfile2\n");exit(0);}while(!feof(out))fputc(fgetc(out),in);fclose(in);fclose(out);}
①
程序实现在屏幕上显示磁盘文件file1.txt的内容
②
程序实现将磁盘文件file2.txt复制到磁盘文件file1.txt
③
程序实现将两个磁盘文件file1.txt和file2.txt的内容合二为一
④
程序实现在屏幕上显示磁盘文件file2.txt的内容