This is a street crossing. There are red and green lights at each corner. Drivers must watch the lights carefully.
    When there is a green light, the cars may go. When there is a red light, the cars must stop. They must wait until the red light changes to green. Then they can go on. Sometimes the cars want to make a right turn or a left turn. They can make a right turn when the light is green or red. But they must wait until the green light is shining if they want to turn left.
    Some people are color-blind(色盲). They can not tell the difference(不同) between the red and green lights. These people must not drive, or there will be accidents. We must keep our streets safe.