如图所示，质量为m的小球固定在杆的一端，在竖直面内绕杆的另一端做圆周运动，当小球运动到最高点时，瞬时速度v= $\sqrt{gR}$ ，R是球心到O点的距离，则当球运动到最低点时对杆的作用力是（   ）

退休护士肖玉梅是南昌章金媛爱心奉献团的志愿者。她经常上门帮助一些需要帮助的人。有人说：“你放着挣钱的事不去干，去干不挣钱的事，太傻了。”但她笑着说：“能和志同道合的姐妹们一起做对社会有意义的事，即使是‘傻’也‘傻’得很快乐。”肖玉梅的话告诉我们（  ）

东北地区的地形以（    ）为主。

根据短文内容和所给汉语提示，写出短文空白处各单词的正确形式,每空限填一词。
   My friend Nancy is ____(15) years old. She is a clever girl in my class. She comes from  ____ (日本).But she likes China very much. Now she ____  (居住) in Shanghai with her parents. Her house is not in the  ____(中心)of the city but she enjoys it. There ____( 有 )a river through her neighborhood.____ (在对面) from her building is an old park.____ （如果) she is tired, she can ____(散步 ) in it to relax herself. She says she wants to be a ____ (女警察) though sometimes it is kind of ____ (危险的)