一级重点 Seeing red Means Danger Ahead (C级) 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 don't 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 they're 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. "It's a really simple detection method," says Nancy Sottos, one of the scientists who worked on the project. "We're 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 mechanophore's 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