A、A speed limit. B、A speed-cheek. C、A distance measurement. D、A traffic-sign. BWhat does "a speed trap" mean in the passage?

Brighton is a popular seaside town on the south coast of England. Not long ago some policemen at Brighton were very puzzled. There had been several serious accidents caused by motorists driving too fast. The police started to set up a speed trap. They measured a distance of 88 yards on a straight road and watched to see how long it took a car to travel that far. They knew that if a car took less than six seconds, it was traveling faster than the speed limit of 30 miles an hour.
    When the policemen were ready, they hid behind a hedge and started to time passing vehicles. During their first half an hour, they caught five drivers. The policemen wrote down the number of each car and the name and address of the driver. But for the next half an hour the policemen did not see anybody driving too fast. They thought that this was very strange. One of them drove a quarter of a mile along the road and saw two students sitting on the grass. They were homing up a sheet of cardboard so that motorists could see it. On the notice one of the students had written: "Danger. Speed Trap."
    The policemen took the notice away and wrote down the names of the students. Later on they were each fined ￡5 for trying to stop the policemen from catching motorists who were breaking the law.