[单项选择]
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 i
A. together
B. behind
C. down
D. apart