TASK（篇章主旨）It's well known to car drivers that red means “stop”， green means “go”， and yellow means “hurry up and make that damn light”. Why those colors, though? The answer lies in the fact that the earliest traffic signals were designed for trains, not cars. They were red and green, gas­powered, and more than a little dangerous in the event of a leak.Red is an inherited (继承的) symbol from railroads, which symbolizes danger in many cultures. As red has a longer wavelength than any other color on the visible spectrum (光谱), it can be seen from a greater distance than other colors. Red has meant “stop” since long before cars existed, with train signals' use of red dating back to the days when mechanical arms lifted and lowered to indicate whether the rail ahead was clear.Green's wavelength is next to yellow's on the visible spectrum, meaning it's still easier to see than any color other than red and yellow. Back in the early days of railway lights, green meant “caution”， while the “all­clear” light was, well, clear or white. It is said that several disastrous collisions happened after an engineer mistook stars in the night for an all­clear. Thus, green became “go”， and for a long time, railways used only green and red to signal trains.From the earliest days of motoring up until the mid­1900s, not all stop signs were red — many were yellow, because at night it was all but impossible to see a red stop sign in a poorly lit area. In 1915 the yellow stop­sign craze began in Detroit, a city that five years later installed its first electric traffic signal, which happened to include the very first yellow traffic light.But what of those mysterious yellow stop signs? As materials and technologies evolved, the ability to produce highly reflective signs meant that red could resume its natural spot in the sign hierarchy, leaving the still­highly­visible yellow to the domain (领域) of“caution”. That's why school zones and buses, crosswalks, and other important warnings are yellow today.Q：What does the passage mainly talk a

A. The significance of road traffic signs.
B. The history of railroad transportation.
C. The symbolic meaning of different colors.
D. The origin and development of traffic lights.