input: Write a python function to flatten a 2D list into a 1D list.
  assert flatten_list([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert flatten_list([[1, 2], [3, 4], [5, 6]]) == [1, 2, 3, 4, 5, 6]
  assert flatten_list([[1], [2], [3]]) == [1, 2, 3]
output: def flatten_list(two_d_list):
      return [element for sublist in two_d_list for element in sublist]