input: Write a function to determine the sublist with the greatest number of elements.
  assert longest_sublist([[0], [1, 3], [5, 7], [9, 11], [13, 15, 17]])==(3, [13, 15, 17])
  assert longest_sublist([[1], [5, 7], [10, 12, 14,15]])==(4, [10, 12, 14,15])
  assert longest_sublist([[5], [15,20,25]])==(3, [15,20,25])
output: def longest_sublist(lst):
      max_length = 0
      max_sublist = []
      for sublist in lst:
          if len(sublist) > max_length:
              max_length = len(sublist)
              max_sublist = sublist
      return (max_length, max_sublist)