下列程序是二分法的程序，横线处应该填上（ ）。
def binary_search(arr, target):
left = 0
right = len(arr) - 1
while left <= right:
mid = (left + right) // 2
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return -1
if arr[mid] == target:
return mid
elif arr[mid] > target:
right = mid - 1
if arr[mid] == target:
return mid
elif arr[mid] > target:
right = mid
else:
left = mid
if arr[mid] == target:
return mid
elif arr[mid] > target:
right = mid - 1
else:
left = mid + 1
if arr[mid] == target:
return mid
else:
left = mid + 1