def fill_columns(columns,board):
    """Returns list of columns of the board.
    >>> fill_columns(['', '', '', '', '', '', ''],['***21**', '412453*', '423145*',\
 '*543215', '*35214*', '*41532*', '*2*1***'])
    ['*44****', '*125342', '*23451*', '2413251', '154213*', '*35142*', '***5***']
    """
    for i in range(len(columns)):
        for row in board:
            columns[i]+=row[i]
    return columns