Seeing red Means Danger Ahead
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
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