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A．Because plant foods lower your risk of chronic disease; animal foods increase it.B．B
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A．They are social workers.B．They are in the medical profession.C．They are coal miners.
听力原文： From the earliest days, people have wanted to keep some record of their achievements. Thus they build monuments, or they tell stories from parents to children, or they write things down. To give shape to the stories, to show how events related to one another, ancient people began to write what we call histories. In both Greek and Latin, the word "historia" means a story or a narrative. Historians were regarded as those who told the story of the past.
One of the earliest historians, a Greek named Herodotus, always tried to make sure that information was as accurate as possible. Herodotus lived in the 400's B.C. He wrote a detailed account of the Persian Wars, the great struggle between the Greeks and the Persians that had taken place in the years before he was born.
A great deal of our information about these wars exists only because Herodotus found it out and wrote it down. Here, to give one example, is his account of how the battle of Salamis began. This was a naval battle at which the Athenian ships defeated the Persians that had taken place in the year 480 B.C. It was one of the turning points of the war, and the question was how the Greeks, badly outnumbered, dared to fight. Here is how Herodotus explained it:
Themistocles, the Athenian leader, told them to board their ships, whereupon the Greeks put to sea. The fleet had hardly left shore when they were attacked by the Persians. At once, most of the Greeks began to retreat, and were about to touch land again when one of the Athenian captains shot forward and charged one of the enemy. The two ships became entangled, and could not be separated. At this the rest of the Greek fleet came up to help, and the battle began.
Ever since Herodotus, historians have recovered and preserved information about past events that otherwise would have been lost. Modern historians have continued to look for new information which helps us understand the past more clearly and more accurately. The usefulness of historians and their works has been recognized by a number of American Presidents, who have made professional historians part of their White House staffs.
15.Which of the following is NOT mentioned as one of the ways people used to keep record of their achievements？
16.Which of the following is NOT true about Herodotus？
17.How did the Greeks dare to fight according to Herodotus？
18.In what way do American Presidents recognize the usefulness of historians and their works？
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In what part of the eye are the rods and cones located？
Why roses are red and violets are blue？
Did you ever wonder why you see the colors you do or if other animals see the same colors that you see？ We see light that bounces off of things around us. When the light enters our eyes, special cells tell our brains about the light. These cells are called photoreceptors. Light is made of little bits called photons. When the sun shines, trillions and trillions of these little bits of light fall on the earth. The photons bounce off of almost everything and some of them enter our eyes. Those bits that enter our eyes allow us to sec. So, where does the color come from？
Starting in the 1600s with Sir Isaac Newton, scientists have believed that there are different kinds of photons. Different types give rise to our sense of colors. The different photons are said to have different wavelengths. Sunlight contains all the different wavelengths of photons. The visible wavelength colors can be seen when you look at a rainbow. Raindrops acting as natural prisms produce the colors.
How do our photoreceptors work？
We have two main types of photoreceptors called rods and cones. They are called rods and cones because of their shapes. These cells are located in a layer at the back of the eye called the retina. Rods are used to see in very dim light and only show the world to us in black and white. This is why you see only black and white when you are outside in the evening or in a dimly lit room. The other type of photoreceptors, the cones, allow us to see colors. They arc not as sensitive as the rods so they only work in bright light. There are three types of cones, one for each of the three main colors we see, red, green and blue.
Some people have a genetic defect that makes one or more of the cones fail. This condition is known as color deficiency. You may have heard it called color blindness. Color blindness is fairly common, affecting about nine percent of all humans. It is much more common in men than in women. To test for color blindness a special picture called an Ishihara test is used.
What about other animals？ What kind of colors do they see？ Most animals see fewer colors than we do, but some see more! We know this by looking at how many kinds of cone photoreceptors they have. Another good indication of what an animal can see is by looking at their own colors. The colors of their prey are also an indication of an animals ability to see color.
What three primary colors do we see？
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