Giải thích câu tục ngữ Uống nước nhớ ngồn

Hey, i dont see it either

Which of the following is an example of an opposing signal? no examplthates given.   i would say, that traffic lights in a city at major intersections direct the flow of traffic in opposite directions so would be an example.also, stop signs on either side of the same street would be an example. at a train crossing, the light for allowing car traffic in opposite directions or not would be another example.

Ithink the answer is d.