Things similar continuously happened in Haiti and Chile．Thousands of people were

buried in the earthquake and lost their fives．What if we could have warned them? People are always trying hard to find a way of preventing building collapsing(坍塌)．Better materials and technology help，but are not a solution．Just like humans，a building has its own life circle from  “birth” to “death”．If we know when a building is going to collapse，we can repair it in advance or get out before it falls．

    Now, scientists at the University of Illinois ha、，e developed a material that turns red before it breaks．The invention could be used in things like climbing ropes，or bridge supports．The secret behind the color-changing material is a type of molecule(分子)．A molecule is a group of atoms held together by chemical bonds(化學(xué)鍵)．Imagine you and your friends standing in a circle，holding hands．Each person stands for one atom，your hands represent the bonds，and the entire circle represents a molecule．If one person lets go of his or her hands，the molecule changes color．The research team put the molecule into a soft material．When the researchers stretched(拉長) the material：it turned bright red a few seconds before it broke into two pieces．

When they repeatedly stretched and relaxed the material，without breaking it，it only turned a little red．

The major problem is，light can get rid of the red color．When the team shone a bright light on the molecule，the broken bond was fixed，and the color disappeared．If bright fight keeps the red color from appearing，the material’s warning system will be useless．Scientists still have a lot of work to do before the color-changing molecule can be used outside the lab．

49. The passage mainly about

A. a way of preventing building from breaking down

B. a series of earthquakes in Haiti and Chile

C. a way of warning before buildings collage

D. a newly-discovered structure of a molecule

50．According to the passage，we call conclude that

A．there are problems to solve before putting the new material to use

B．scientists have found the life circle of buildings from “birth” to “death”

C．bright fight makes the red color visible to human eyes

D．the new material is unlikely to be used in building bridges

51．The color-changing molecule can

A．make building materials much stronger

B．help building materials get stretched if necessary

C．prevent the earthquake from killing people

D．remind people to leave buildings before they fall

52．The warning system is based on the fact that

A．red color won’t show up when it meets bright fight

B．a(chǎn) material with a certain type of molecule Can turn red when s廿etched

C．a(chǎn) building has its own life circle of “birth”to“death”

D．the broken bond will fix itself when a molecule meets bright fight

49．C 50．A 51．D 52．B

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