What are some reasons as to why communism hasn't been successful in history?

Steven spielberg's classic movie jaws was a thriller about a great white shark that terrorized tourists at a local beach. just before the shark's appearance, eerie music began playing. as the unseen shark came closer, the tempo of the music picked up. after the audience had experienced this a few times, the sound of the music triggered the emotional reaction of fear in the audience even though the shark still had not appeared. at that point, the sound of the eerie music was a: a) unconditioned response (ucr) b) conditioned response (cr) c) conditioned stimulus (cs) d) unconditioned stimulus (ucs)