题目内容：

The color red often means danger --- and by paying attention, accidents can be prevented. At railroad crossings, flashing red fights warn cars to stay back. A red light at a traffic crossing tells cars to stop, so they don't run into other cars.

In the future, the color red also may help prevent danger at 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 before a structure collapses(倒塌) or falls apart.

The secret behind the color-changing material is a particular type of molecule(分子). Molecules come in all shapes and sizes, and make up everything you can see, touch or feel. How a molecule behaves depends on what kinds of atoms(原子) it contains, and how they're held together.

To get a rough picture of one way atoms are held together in a molecule, imagine you and your friends standing in a large circle, holding hands. Each person represents one atom, your clasped hands represent the bond, and the entire circle represents a molecule.

The molecule being used to turn the material red is called mechanophore(机械响应性聚合物). When one chemical bond in the mechanophore molecule breaks, the rest of the molecule turns red.

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 disappears. This "self-healing" may be a problem for engineers who want to use the color-changer in big construction projects that will be outside, in the sunlight. And if bright light keeps the red color from appearing, then the mechanophore's warning system will be useless.

Scientists still have a lot of work to do before the color-changing molecules can be used outside the lab. If mechanophore can be used in the real world, they are suggested to be employed in a new kind of paint.

1.The first paragraph mainly tells us __________.

A. the color red is widely used in our life

B. how to prevent car accidents

C. the color is an important signal of warning

D. how to use color red

2.The color red will be used at construction sites to _________.

A. tell different materials used in the building

B. warn people that the building is dangerous

C. make the building more beautiful

D. tell people the building is under construction

3.The example given in the fourth paragraph is to show _________.

A. the way atoms are held together in a molecule

B. how a molecule behaves in an object

C. how to play an old game

D. how people get along with each other

4.What can make the mechanophore’s warning system ineffective?

A. Paints       B. Water      C. Poor building materials      D. The bright light