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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 extract_keywords_from_sentences(self, sentences):
 """Method to extract keywords from the list of sentences provided. :param sentences: Text to extraxt keywords from, provided as a list of strings, where each string is a sentence. """

        phrase_list = self._generate_phrases(sentences)
        self._build_frequency_dist(phrase_list)
        self._build_word_co_occurance_graph(phrase_list)
        self._build_ranklist(phrase_list)