Question: The positions of the feet in ballet is a fundamental part of classical ballet technique that defines standard placements of feet on the floor, two additional positions, known as the sixth and seventh positions, were codified in the 1930s by Serge Lifar, who was a French ballet dancer and choreographer of Ukrainian origin, famous as one of the greatest male ballet dancers of which century?
Sentence (from "Royal Ballet School"): The Royal Ballet School is one of the world's greatest centres of classical ballet training.
Relevant evidence: No