下列归并算法程序中，横线处应该填入的是（    ）。

def merge_sort(arr):

      if len(arr) <= 1:

      return arr

      mid = len(arr) // 2

      left = arr[:mid]

      right = arr[mid:]

      merge_sort(left)

      merge_sort(right)

      return merge(left, right)

def merge(left, right):

      result = []

      i, j = 0, 0

      ———————————————

            if left[i] < right[j]:

                  result.append(left[i])

                  i += 1

            else:

                  result.append(right[j])

                  j += 1

      result += left[i:]

      result += right[j:]

      return result