A distance of approx. 100 meters on approach to a level crossing

What is the meaning of the following traffic sign?

1. If you see the train’s lights, stop six meters before the crossing.
2. A distance of about 170 meters to an approaching level crossing.
3. Correct answer: A distance of approx. 100 meters on approach to a level crossing.
4. A distance of about 250 meters to an approaching level crossing.