题干

下列关于探索DNA是遗传物质实验的相关叙述,正确的是(  )

A:肺炎双球菌R型转化为S型是基因突变的结果

B:格里菲思实验证明了DNA是肺炎双球菌的遗传物质

C:赫尔希和蔡斯实验中T2噬菌体的DNA可以用32P直接进行标记

D:赫尔希和蔡斯实验证明了DNA是T2噬菌体的遗传物质

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答案(点此获取答案解析)

D

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阅读下面材料,从每题所给的A、B、C、D四个选项选出最佳选项。

   Once, Louis the Twelfth, the King of France, happened to walk into the kitchen of his palace. There he found a small boy busy at work and singing happily.

   The boy had bright eyes, and a happy, sunny face, and his looks and manners pleased the king. With a smile Louis asked him his name. He replied that his name was Simon. In answer to further questions, the king learnt that the boy was an orphan; both his father and mother were dead.

   “Are you content(满足的)to do this kind of work?” the king asked, pointing at the bowls which the boy was washing. “Are you willing to spend your time in the kitchen when so many of the others work upstairs?”

   “Why not?” answered the boy, with a twinkle in his eye. “I am doing as well as the best of them. The king himself can do no better.”

   “Indeed!” said the king, no little surprised at the boy's words. “How do you know that?”

   “Well, sir, the king lives, and so do I. I am content with my life. Can the king say that he is content with his life?”

   Louis walked away, his mind full of strange, sad thoughts. No one knew better than he did that kings seldom feel content.

   The next day, much to Simon's surprise, he was called before the king. He was still more surprised when he found that his visitor of the day before was Louis himself. The king talked with him about many things, and found him to be a very quick and clever boy.

   After that Simon was made to serve the king directly. Step by step, he rose from one important task to another, until he became one of the best soldiers of that time. He is known in his history as General La Roche, and considered as one of the greatest men of France.

   If Simon had not, early in life, learnt to be content with his life, he might never have been given those chances. The king did not want people who were always unhappy because they did not have as good a place as some others. You cannot too soon learn that only those who are content are happy.