def biggest(aDict):
    '''
    aDict: A dictionary, where all the values are lists.

    returns: The key with the largest number of values associated with it
    '''
    maior = 0
    chave_maior = None
    for chave, valor in aDict.items():
        if len(valor) > maior:
            maior = len(valor)
            chave_maior = chave

    return chave_maior


animals = { 'a': ['aardvark'], 'b': ['baboon'], 'c': ['coati']}

animals['d'] = ['donkey']
animals['d'].append('dog')
animals['d'].append('dingo')


print(biggest(animals))