def mi_rank(score):
    """Rank the score with a letter:

        * A if :math:`\\text{score} > 19`;
        * B if :math:`9 < \\text{score} \\le 19`;
        * C if :math:`\\text{score} \\le 9`.
    """
    return chr(65 + (9 - score >= 0) + (19 - score >= 0))
