完形填空The color red
The color red often means danger —— and by paying attention， 1 can be prevented. At railroad crossings， flashing red lights warn cars to stay back. A red light at a traffic intersection tells cars to stop， so 2 dont run into other cars.
In the future， the color red also may help prevent danger 3 construction sites. Thanks to new work by engineers， bridge supports — or other kinds of materials — could one day contain a color-changing material. It will turn red 4 a structure collapses or falls 5 。 A tiny molecule may make a big difference in future warning systems.
A polymer 6 a color-changing molecule called a mechanophore turns red seconds before it snaps. The technology may one day allow damage to materials or structures4 to be easily 7 。
The secret behind the color-changing material is a particular type of molecule. A molecule is a group of atoms held together by 8 bonds. Molecules come in all shapes and sizes， and make up 9 you can see， touch or feel. How a molecule behaves depends on what kinds of atoms it contains， and how theyre held together.
When a polymer containing a color-changing molecule called a mechanophore is about to breaks， it produces a color. When a polymer with mechanophore molecules becomes "injured" or 10 ， one of the mechanophore bonds breaks and the material turns red. "Its a really simple detection method，" says Nancy Sottos， one of the scientists who worked on the project. "Were 11 up this one bond， and it changes color. " Sottos and her team tested the color-changing polymers in their lab. The test 12 proved encouraging.
There is a way to get rid of6 the red color： 13 。 When a bright light is shone on the mechanophore， the broken bond is fixed7- and the red color disappears. This "self-healing" may be a problem for engineers. They need to use the color-changer in big construction projects that will be 14 ， in sunlight. And sunlight will make the mechanophores warning system useless.
Sottos and her fellow scientists still have 15 work to do before the color-changing molecules can be used outside the lab.
练习：
1. A measures B accidents C actions D collap-ses
2. A they B it C some D most
3. A with B over C at D in
4. A before B after C once D while
5. A together B behind C down D apart
6. A contacting B conducting C containing D considering
7. A controlled B spotted C repaired D changed
8. A technical B electronic C physical D chemical
9. A everything B something C nothing D anything
10. A weak B strong C tough D soft
11. A using B opening C turning D finishing
12. A laws B theories C tools D results
13. A air B electricity C light D sound
14. A aside B beside C inside D outside
15. A a part of B a pair of C a piece of D a lot of
答案：BACAD CBDAA BDCDD
看见红色意味着危险在前方
红色常常表示危险，通过引起注意能够防止意外。在铁路交叉口，闪烁的红灯警告车辆靠后。在交通十字路121的红灯告诉车辆停止前进，所以就不会发生车祸。
在未来，建筑工地的红色可能也可以防止危险。多亏了工程师的新作和桥梁支柱等其他的材料，它们有一天能包含一种会变换颜色的物质，该物质能够在建筑物倒塌或之前变成红色。一个小小的分子就会对报警系统起到很大影响。
一个聚合物包含一种变换颜色的分子叫做机械响应性聚合物，它能够在在聚合物断裂前几秒钟变成红色。这项技术能够很容易地发现材料或建筑物的损坏。
这个变换颜色的物质的秘密就是一种特别的分子。一个分子是由化学键把一群原子组合起来。分子有各种各样的形状和大小，所以能够组成你所见到的、摸到的和感觉到的任何东西。一个分子怎样表现取决于它包含什么样的原子，和他们被怎样组合起来。
当包含变换颜色分子的机械响应性聚合物要断裂的时候，它会产生一种颜色。当一个包含机械响应性聚合物分子的聚合物“受损害”或者变弱的时候，其中的一个机械响应性聚合物键破碎，这个物质变成红色。“这是一个很简单的检测方法。”Nancy Sottos说，她是为这个项目工作的科学家之一。Sottos和她的团队在他们的实验室测试这种变换颜色的聚合物，测试的结果非常鼓动人心。
有一种方法可以除去红色：光。当强光照射在机械响应性聚合物上，断裂的键被修复——红色就会消失。这种“自我修复”或许给工程师们带来问题。他们需要在露天的，也就是暴露在阳光下的大建筑工程外面使用换色器。阳光会使机械响应性聚合物的报警系统不起作用。
要让这些变色分子在实验室以外也能使用，Sottos和她的同事们还有许多工作要做。