def letter_to_color(letter):
    """
    Transfer a letter into a color
    A corresponds to red. B corresponds to yellow. C corresponds to green. D corresponds to blue.
    :param letter: an input letter
    :type letter: string
    :return: a color
    :rtype: string
    """
    if letter == "A":
        return "rgb(230,0,0)"
    elif letter == "B":
        return "rgb(252,251,52)"
    elif letter == "C":
        return "rgb(0,175,0)"
    elif letter == "D":
        return "rgb(25,128,255)"