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阅读理解。阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。 About ten men in every hundred suffer from color blindness in some way. Women are luckier，only about one in two hundred is affected in this matter. Perhaps，after all，it is safer to be driven by a woman. There are different forms of color blindness. In some cases a man may not be able to see deep red. He may think that red，orange and yellow are all shades of green. Sometimes a person cannot tell the difference between blue and green. In rare cases an unlucky man see everything in shades of green—a strange world indeed. Color blindness in human beings is a strange thing to explain. In a single eye there are millions of very small things called “cones”．These help us see in a bright day and tell the difference between colors. There are also millions of “rods”， but these are used for seeing when it is nearly dark. They show us shape but no color. Some insects have favorite colors. Mosquitoes(蚊子) like blue but do not like yellow. A red light will not attract insects，but a blue lamp will. In a similar way human beings also have favorite colors. Yet we are lucky. With the aid of the cones in our eyes we can see many beautiful colors by day，and with the aid of the rods we can see shapes at night. One day we may even learn more about the invisible colors around us. . This passage is mainly about ________. | 
