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. 32. 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.

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

　　A. suffering a lot

　　B. collecting beyond imagination

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