抢答器的实现，当按下任一个按键后，其他的按键都没有效果，系统自动只识别接收到的第一个信号，并在数码管上显示出来，重新启动后开始下一轮抢答-Responder realization, when you press any button, the other buttons have no effect, the system automatically recognizes only the first received signal and the digital tube display, start the next round after restart Responder