A31211. 下列二分枚举算法中，{ }处应该填入的程序是（{}不算做程序的一部分）（ ）。def binary_search(arr, x): low = 0 high = len(arr) - 1 while low <= high: { } return -1

```Python

mid = (low + high) // 2

      if arr[mid] == x:

            return mid

      elif arr[mid+1] > x:

            high = mid - 1

      else:

            low = mid + 1

```Python

mid = (low + high) // 2

      if arr[mid] != x:

            return mid

      elif arr[mid+1] > x:

            high = mid - 1

      else:

            low = mid + 1

```Python

mid = (low + high) // 2

      if arr[mid] == x:

            return mid

      elif arr[mid] > x:

            high = mid - 1

      else:

            low = mid + 1

```Python

mid = (low + high) // 2

      if arr[mid] ！= x:

            return mid

      elif arr[mid] > x:

            high = mid - 1

      else:

            low = mid + 1