读一读，回答下列问题。
    Betty is very happy because she moves to her new home today. Her new home is very big and nice. It has three bedrooms. The big one is for her parents. The small one is for her brother Paul and the middle one is for her. Betty likes pink，so her bedroom is a pink room. The walls are pink. The curtains(窗帘)are pink. The bed and wardrobe(衣柜)are pink, too. Every day, Betty does her homework in her bedroom. Betty's father likes the new home, too. He likes the study because it's quiet. He can read in it. Betty's mum likes the kitchen. It is big and clean. She can cook nice food for the family. Paul likes the new home because the living room is so big and he can play games with his friends there.

下列现象中属于光的直线传播的是 （   ）

如图所示，轨道ABC光滑，弹簧固定在水平轨道末端，小球从A处由静止滚下，撞击弹簧后又将沿水平轨道返回，接着滚上斜面．在这整个过程中，机械能转化的情况是（　　）

​

阅读文言文，完成各题。

       锲而不舍，朽木不折；锲而不舍，金石可镂。蚓无爪牙之利，筋骨之强，上食埃土，下饮黄泉，用心一也。蟹六跪^①而二螯^②，非蛇鳝之穴无可寄托者，用心躁也。

      【注释】①跪蟹脚。②螯蟹钳。

如图，矩形ABCD的边AB与x轴平行，顶点A的坐标为（2，1），点B与点D都在反比例函数y= $\frac{6}{x}$ （x＞0）的图象上，则矩形ABCD的周长为____．