What should you do if you encounter a railroad crossing with no warning devices?

Answer: A — Come to a full stop

When you encounter a railroad crossing with no warning devices, it is important to come to a full stop. This is because there may be a train approaching and without warning devices, you cannot be sure if it is safe to proceed. Option B to look both ways and proceed may be tempting, but it is not safe because you may not have enough time to react if a train is coming. Option C to drive slowly and proceed carefully is also not safe as it does not guarantee that you will be able to stop in time if a train suddenly appears. Option D to change direction and drive away is also not a safe option as it may lead you to an unfamiliar or unsafe area. Therefore, the best course of action is to come to a full stop and wait for a clear signal before proceeding.