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Two men took on the job of human traffic signals at an intersection in Shijiazhuang, Hebei province, when the electric traffic signals broke down and several traffic accidents occurred over the past month. They set up temporary signals using large plastic boards with the words "stop" and "go", turning in one direction or another every 30 seconds to aid drivers. The city was evaluating the problem with the lights and sent police officers to direct rush-hour traffic. How often did the human traffic lights change direction? A. Every 20 seconds B. Every 30 seconds C. Every 40 seconds | 
