人与其他动物、植物等生命相比，其突出的表现是（  ）

如图所示，圆柱O₁O中，母线AB与底面垂直，BC是⊙O的直径，点D是⊙O的圆周上异于B，C的点．

（1）求证：平面ABD⊥平面ADC；

（2）若BD=2，CD=4，AC=6，求圆柱O₁O的表面积．

   Mr. Li worked in a post office in Weihai. Last month he retired. His son works in America. Five years ago Mrs. Li went to America to look after their grandson. So Mr. Li decided to see them.
   Mr. Li was afraid of taking the plane so he decided to go to America by ship. On the first morning on the ship, Mr. Li met an American. The man said to him, “Good morning!” Mr. Li thought the man told him his name. So he said to him, “Li Haitao.”
   On the second morning, Mr. Li met his son’s friend on the ship. He told him about the American. After hearing that, his son’s friend laughed and said, “He said hello to you in the morning.”
   On the third morning when Mr. Li met the American again, he said, “Good morning!” And the American said to him, “Li Haitao, sir.”

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