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