单项选择题

Seeing Red Means Danger Ahead
The color red often means danger-and by paying attention，accidents can be prevented．In the future，the color red also may help (51) danger at construction sites．Thanks to new work by engineers，bridge supports-or (52) kinds of materials-could one day contain a color-changing material．It will turn red (53) a structure collapses or falls apart．
The secret (54) the color-changing material is a particular type of molecule(分子)．A molecule is a group of atoms(原子)held together by chemical bonds．Molecules come (55) all shapes and sizes，and make up (56) you can see，touch or feel．How a molecule behaves depends on (57) kinds of atoms it contains，and how they’re held together．
When a polymer(聚合物)containing a color-changing molecule called a mechanophore(机械响应性聚合物)is about to break，it produces a (58) ．When a polymer with mechanophore molecules becomes“injured”or (59) ，one of the mechanophore bonds (60) and the material turns red
“It’s a really simple detection method，”says Nancy Sottos，one of the scientists who worked on the project．Sottos and her team tested the color-changing polymers in their lab．The test (61) proved encouraging．
There is a way to get rid of the red color：light．When a bright light is shone on the mechanophore， the broken bond is fixed-and the red color (62) ．This“self-healing”may be a problem for engineers．They need to use the color-changer in big construction projects that will be (63)，in sunlight．And sunlight will make the mechanophore’s warning system (64) ． Sottos and her fellow scientists still have (65) work to do before the color-changing molecules can be used outside the lab．

63（）

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