如图，∠ACB=60°，⊙O的圆心O在边BC上，⊙O的半径为3，在圆心O向点C运动的过程中，当CO=____时，⊙O与直线CA相切．

材料一  突然之间......有什么东西发出的声响，画面在抖动，你简直不敢相信自己的眼睛......我们面前沸腾着奇妙的生活......突然之间，它消失了。我们面前呈现......一块白色的银幕，看来上面什么都没有了。

                                              ——高尔基

材料二  1960年，美国总统选举，总统候选人辩论通过电视向全国转播。民主党候选人肯尼迪年轻有为、整洁干练，给电视观众留下了比对手更好的印象，而收音机听众却感觉其对手即尼克松表现更佳。

    When you take a walk in any of the cities in the West, you can see a lot of people ____(walk) with dogs. It is still true that a dog is the most useful and faithful animal in the world, but the reasons why people keep a dog ____(change). In the old days people used to train dogs to protect themselves against attacks by other beasts. And ____(late) they came ____(realize) that a dog was not only useful for protection,____ willing to obey his master. For example, when people used dogs for hunting, the dogs would not eat what was caught without ____(permit). But now people in the city need not protect themselves against attacks of animals. Why do they keep dogs, then? Some people keep dogs to protect themselves from robbery. But the ____(important) reason is for companionship. For a child, ____ dog is his best friend when he has no friends to play with. For young couples, a dog is ____child when they have no children. For old couples, a dog is also their child when their real children have grown up. So the main reason why people keep dogs has changed from protection ____ friendship.