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.
練習：
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
看見紅色意味著危險在前方
紅色常常表示危險，通過引起注意能夠防止意外。在鐵路交叉口，閃爍的紅燈警告車輛靠后。在交通十字路121的紅燈告訴車輛停止前進，所以就不會發生車禍。
在未來，建筑工地的紅色可能也可以防止危險。多虧了工程師的新作和橋梁支柱等其他的材料，它們有一天能包含一種會變換顏色的物質，該物質能夠在建筑物倒塌或之前變成紅色。一個小小的分子就會對報警系統起到很大影響。
一個聚合物包含一種變換顏色的分子叫做機械響應性聚合物，它能夠在在聚合物斷裂前幾秒鐘變成紅色。這項技術能夠很容易地發現材料或建筑物的損壞。
這個變換顏色的物質的秘密就是一種特別的分子。一個分子是由化學鍵把一群原子組合起來。分子有各種各樣的形狀和大小，所以能夠組成你所見到的、摸到的和感覺到的任何東西。一個分子怎樣表現取決于它包含什么樣的原子，和他們被怎樣組合起來。
當包含變換顏色分子的機械響應性聚合物要斷裂的時候，它會產生一種顏色。當一個包含機械響應性聚合物分子的聚合物“受損害”或者變弱的時候，其中的一個機械響應性聚合物鍵破碎，這個物質變成紅色。“這是一個很簡單的檢測方法。”Nancy Sottos說，她是為這個項目工作的科學家之一。Sottos和她的團隊在他們的實驗室測試這種變換顏色的聚合物，測試的結果非常鼓動人心。
有一種方法可以除去紅色：光。當強光照射在機械響應性聚合物上，斷裂的鍵被修復——紅色就會消失。這種“自我修復”或許給工程師們帶來問題。他們需要在露天的，也就是暴露在陽光下的大建筑工程外面使用換色器。陽光會使機械響應性聚合物的報警系統不起作用。
要讓這些變色分子在實驗室以外也能使用，Sottos和她的同事們還有許多工作要做。