def merge_tweets(labels_tweets):
    """
    Merge tweets for each cluster.
    labels_tweets: DataFrame
    return: a dict of lables and their text
    """
    pass
    labels = set(labels_tweets['label'])
    labels_text = dict()
    for label in labels:
        qry = "label == {}".format(label)
        res = labels_tweets.query(qry)
        tweets = list(res['tweet'])
        text = " ".join(tweets)
        labels_text[label] = text
    return labels_text