def create_empty_board(n):
   """
   Creates and returns a grid (list of lists), that has n
   rows and each row has n columns.  All the elements in
   the grid are set to None.
   >>> create_empty_board(2)
   [[None, None], [None, None]]
   """
   grid = []                   # Create empty grid
   for y in range(n):          # Create rows one at a time
       row = []
       for x in range(n):      # Build up each row by appending to a list
           row.append(None)
       grid.append(row)        # Append the row (list) onto grid
   return grid