Have you always wondered why traffic lights have these colors — green, red and yellow? To find out why these colors were chosen, it would be helpful to know where traffic lights (and their colors) come from.

In 1910, the first traffic signals were installed. Officers were using whistles and lights to maintain the flow of cars by letting drivers know when to stop or go. In 1920, William Potts created the first traffic light. At that time, there were no rules about the kind of lights or patterns that had to be used. And so, these lights would look different everywhere you went. In 1935, the Federal Highway Administration set up the “Manual on Uniform Traffic Control Devices” (《统一交通控制设施手册》) that held standards for all signs and signals. That manual also required the traffic lights to be red, yellow and green everywhere.

So, where did the specific colors come from? Before the traffic lights, trains already used light signals to help with the flow of train traffic. In these traffic lights red meant stop, white meant go and green meant “proceed with caution”. When the color white caused issues (it could be mistaken for something else by train drivers, causing accidents), they moved from white to green for the sign for “go”. And because yellow was more distinguishable, they made yellow the color for “proceed with caution”.

But then why red? The color red is the color with the longest wavelength. And that means that you can see it from further away. And that is very helpful when it comes to warning signs and traffic lights that want you to stop for safety.

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1.What’s the purpose of the first paragraph?

A To ask for information.

B To entertain the readers.

C To lead to the topic.

D To present an idea.

解析：选C。C 推理判断题。根据后文内容可知，第一段是为了引出本文要讨论的话题：为什么交通灯选定了绿色、黄色和红色三种颜色。故选C。

2.The writer develops the second paragraph by __________.

A the order of time

B the order of space

C giving examples

D explaining reasons

解析：选A。A 细节理解题。根据第二段中出现的年份数字“In 1910”“In 1920”“In 1935”可知，本段按照时间顺序进行描述。故选A。

3.The underlined word “issues” in Paragraph 3 refers to __________.

A topics

B problems

C discussions

D results

解析：选B。B 词义猜测题。根据划线部分前文可知，起初火车信号灯使用了红色(停止)、白色(通行)和绿色(小心行驶)三种颜色，但是可能由于火车司机误把白色看成了其他东西而导致了车祸，人们就把颜色进行了变更。即白色表示“通行”时引起了一些问题。故选B。

4.Why does the author write the text?

A To instruct. 

B To inform. 

C To entertain. 

D To comment.

解析：选B。B 推理判断题。本文主要说明了交通信号灯为何会选定绿色、黄色和红色这三种颜色，即本文旨在说明。故选B。