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Why may the repair process of the DNA finally fail？
When I was a little boy, I really liked my older brother, Tom. He was the best pitcher (投手) on his baseball team, the Dreams. I always played catch (接球) with Tom. He threw so fast that I couldn't often catch the ball. He always asked me, "Have you had enough yet, Mark？" But I said I wanted to play more. I wanted to be a good player like Tom.
When I was ten years old, I joined a team, the Stars. I didn't join the Dreams because my friends on the Stars wanted me to join them. I practiced very hard with my team every Saturday afternoon. The next summer came, my team was going to play against the Dreams. I thought, "I want to hit Tom's basketball, but it will be too fast for me to hit." I practiced hitting basket- balls harder than before. I talked with Tom about the game. He said, "With my basketball, my team is going to win."
One Saturday afternoon we had the game. It began at 1: 00. Mother came to watch us. No one could hit Tom's basketball. The game was coming to its end. My last chance came. I had to hit his basketball.
"Strike one," called the umpire (裁判员). "Oh, that's fast!" I thought. Tom looked great.
He threw again, but I couldn't hit that one, either. "Hit the next ball. Mark," the players on my team shouted. I thought, "How can I do that？ No one has hit his basketball since the game began. But I have to hit it for our team."
Tom threw again. It was another basketball. I kept my eyes on it. This time I was able to hit it! I watched the ball in the sky and started to run. A player on the Dreams ran fast to try to catch it, but he couldn't. I was safe! Everyone on my team shouted, "You did it!" I was very happy. But when I saw Tom, I could't be happy any more. He was watching me. He didn't look happy.
The next player on my team didn't hit Tom's basketball. That was the end of the game. The Dreams won.
After the game I went to Tom, but I didn't know what to say to him. He said to me with a smile, "You hit my basketball very well." I said, "Thank you." Tom said to me, "You hit my basketball today, but next time it will be much faster. O. K.？" I answered him with a smile, "Yes, I'll practice much harder, Tom." Mother listened to us and said, "Tom and Mark, you both did a very good job." She looked very happy.
Which of the following is TRUE？
Which of the following is not a possible damage by UV radiation？
Population Densities
The average population density (密度) of the World is47 persons per square mile. Continental densities range from no permanent inhabitants in Antarctica to 211 per square mile in Europe. In the western hemisphere, population densities range from about 4 per square mile in Canada to 675 per square mile in Puerto Rico. In Europe the range is from 4 per square mile in Iceland to 831 per square mile in the Netherlands.
Within countries there are wide variations of population densities. For example, in Egypt, the average is 55 persons per square mile, but 1,300 person inhabit each square mile in settled portions where the land is arable (适合耕种的).
High population densities generally occur in regions of developed industrialization, such as the Netherlands, Belgium, and Great Britain, or where lands are intensively used for agriculture, as in Puerto Rico and Java.Low average population densities are characteristic of most Underdeveloped countries.
Low density of population is generally associated 'with a relatively low percentage of cultivated land.This generally results from poor quality lands. It may also be due to natural obstacles to cultivation, such as deserts, mountains, or malaria-infested (疟疾横行) jungles; to land uses other than cultivation, as pasture and forested land; to primitive methods that limit cultivation; to social obstacles
More economically advanced countries of low population density have, as a rule, large proportions of their populations living :in urban areas. Their rural population densities are usually very low. Poorly developed countries of correspondingly low population density, on the other hand, often have a concentration of rural population living on arable land, which is as great as the rural concentration found in the most densely populated industrial countries.
Which of the following area has the highest average population density？
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