单项选择题

Seeing Red Means Danger Ahead
The color red often means danger and by paying attention, (51) can be prevented. At railroad crossings, flashing red fights warn cars to stay back. A red light at a traffic intersection tells cars to stop, so (52) don’t run into other cars.
In the future, the color red also may help prevent danger (53) 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 (54) a structure collapses or falls (55) . A tiny molecule may make a big difference in future warning systems.
A polymer (56) a color-changing molecule called a mechanophore turns red seconds before it snaps. The technology may one day allow damage to materials or structures to be easily (57) .
The secret behind the color-changing material is a particular type of molecule. A molecule is a group of atoms held together by (58) bonds. Molecules come in all shapes and sizes, and make up (59) 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 (60) , 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 (61) up this one bond, and it changes color." Sottos and her team tested the color-changing polymers in their lab. The test (62) proved encouraging.
There is a way to get rid of the red color: (63) . When a bright light is shone on the mechanophore, the broken bond is fixed on 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 (64) , in sunlight. And sunlight will make the mechanophore’s warning system useless.
Sottos and her fellow scientists still have (65) work to do before the color-changing molecules can be used outside the lab.