A. Take measures to keep safe B. Watch out for selfie

　　C. Ask yourself for excitement D. Take responsibility

　　If we are observant(善于观察的), we can find that most of the flowers in nature are red, orange and yellow. If we have seen a black flower, it is a chance in a million. People have made census (统计) to colors of more than four thousand kinds of flowers and discovered that only eight of them are black. Why are black flowers so rare?

As we know, sunlight is formed by seven different kinds of colored light. The wave length of each light changes, so the quantity of heat in each light changes, too. Flowers, especially their petals (花瓣) , are very weak and easy to the harm caused by high temperature. Black flowers can take in all the light waves, which cause the flowers to dry up in a high temperature. So the black flowers can hardly continue their lives. While red flowers, orange flowers and yellow ones can protect themselves from sunlight by reflecting（反射）the red light, orange light and yellow light, each of which has a large quantity of heat.

That is why red, orange and yellow flowers are very common in nature while black flowers are so unusual.

28. "It is a chance in a million" means______.

　　A. something extremely rare B. something lucky

　　C. something common D. something possible

29. Sunlight is formed by _____.

　　A. many different kinds of colored light

　　B. three different kinds of colored light-red, orange and yellow

　　C. seven different kinds of colored light

　　D. four thousand kinds of colored light

30. Black flowers are so rare because ______.

　　A. they are so weak that it's difficult for them to grow up

B．they can take in the light of all wave length which make the flowers dry up because of high temperature.

C. the petals of black flowers are very delicate（易碎的）