def max_key(my_dict):
    """Returns the key of the maximum value in a dictionary."""

    # there are many ways in Python to do this
    # this one only uses functions that we learned
    # in the lecture

    max_key = None

    for key, value in my_dict.items():
        if max_key == None or my_dict[key] > my_dict[max_key]:
            max_key = key

    return max_key