Question: The Theater des Westens (Theatre of the West) is one of the most famous theatres for musicals and operettas in Berlin, Germany, here Ballets Russes appeared with which Russian prima ballerina of the late 19th and the early 20th centuries, and was  a principal artist of the Imperial Russian Ballet?
{'answer': 'Anna Pavlova', 'supporting_facts': [['Theater des Westens', 3], ['Anna Pavlova', 0]]}