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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def active_classification(keywords, exposure_key):
 """Helper to retrieve active classification for an exposure. :param keywords: Hazard layer keywords. :type keywords: dict :param exposure_key: The exposure key. :type exposure_key: str :returns: The active classification key. None if there is no active one. :rtype: str """

    classifications = None
    if 'classification' in keywords:
        return keywords['classification']
    if 'layer_mode' in keywords and keywords['layer_mode'] == \
            layer_mode_continuous['key']:
        classifications = keywords['thresholds'].get(exposure_key)
    elif 'value_maps' in keywords:
        classifications = keywords['value_maps'].get(exposure_key)

    if classifications is None:
        return None
    for classification, value in list(classifications.items()):
        if value['active']:
            return classification
    return None