def key_of_min_value(d):
    """Returns the key in a dict d that corresponds to the minimum value of d.

    >>> letters = {'a': 6, 'b': 5, 'c': 4, 'd': 5}
    >>> min(letters)
    'a'
    >>> key_of_min_value(letters)
    'c'
    """
    # BEGIN Question 0
    return min(d, key=d.get)
    # END Question 0