程序代码：

clock cpu_just = 0;
char answer = 0;

printf("是否进入电梯？(y/n):");
scanf(" %c",&answer);

if(tolower(answer) == 'y')
{
  cpu_just = clock();
  for((clock() - cpu)/CLOCK_PER_SEC < 25);
  printf("您以进入电梯，请选择要去的楼房:");
  /* ... */
}
else
/* ... */