Thick dust rose and a large building collapsed. This was the scene in Zhu Zhou，Hunan Province on May 17. Nine people were killed and 16 injured.
Things were similar in the Sichuan earthquake. Thousands of people were buried in the ruins and lost their lives. What if we could have warned them?
People are always racking their wits to find a way of preventing buildings’ collapse. Better materials and technology help，but they are not a solution.
Just like humans，a building has its own life cycle from “birth” to “death”．If we know when a building is going to collapse，we can repair it in advance or get out of it before it falls. Now，scientists at the University of Illinois have developed a material that turns red before it breaks. The invention could be used in things like climbing ropes or bridge supports. The research was led by Nancy Sottos，a professor at the university’s Beckman Institute，and Douglas Davis，a graduate research assistant.
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. 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 the material，it turned bright red for 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 that light can get rid of the red color. When the team shone bright light on the molecule，the broken bond was fixed，and the color disappeared. Scientists have yet to do more research before the color—changing molecules can be used outside the lab.
What does the passage mainly introduce?

A．The collapse of a large building in Zhu Zhou, Hunan Province.

B．A type of molecule that can be put into a soft material

C．A material that turns red before it breaks.

D．A way of preventing building from collapsing.

The underlined words “racking their wits” in the third paragraph mean ________.

A．suffering a lot	B．collecting beyond imagination
C．losing a lot	D．thinking very hard

Which of the following is a WRONG statement?

A．Thousands of people died from the collapse of buildings in the Sichuan earthquake because they hadn’t been warned.

B．There is no building that will never collapse.

C．A new material has been applied to buildings to warn people of collapse.

D．A color—changing material can turn red before it breaks.

What can we infer from the passage?

A．The color—changing molecules are certain to be used outside the lab.

B．There will be less collapse with the help of the color—changing material.

C．The bright light may make the material’s warning system useless.

D．The problem caused by bright light will be solved by scientists soon.