运动状态不变的物体包括　____ 　的物体和　____ 　的物体；物体保持运动状态不变的条件是　____ 　或　____ 　．

    I did something wrong today.

    While at the beach this afternoon. I met a boy called Bob We talked a lot. He said that it was fun playing on the cliffs (悬崖). I told him that I was not allowed to go near that place(Mum and Dad had always told me it was dangerous to play there). He called me a coward. I wanted to prove to him that I was just as brave, so I agreed to go with him.

    We wanted to make our way down to explore (探索)the area, so Bob started to climb down and I followed him. Unluckily, some rocks came down after us. One of them nearly hit my head. We could not get back up. We shouted and shouted for help, but for a long time no help came. I was really scared. I wanted to cry. Then, we heard some sounds. Someone had heard us. A woman tied (系 )a rope around herself while a man at the cliff top held the rope. She made her way down to save us.

    My parents were relieved (宽慰的)that I was unhurt, but they were angry that I had done such a risky thing. I apologized and promised that I would not do anything like that again.

投掷一枚均匀的硬币，落地时正面或反面向上的可能性相同．有甲、乙、丙三人做“投硬币”实验，他们分别投100次，结果正面向上的次数为：甲60次、乙40次、丙50次．则下列说法正确的是（　　）

《2002年地球生态报告》显示，所有的自然资源会在2075年之前耗尽。这份报告除评估全球生态系统现状外，还根据每个国家人口数量和各国自给自足能力的平衡关系，计算各国的“生态脚印”大小，换言之，也就是指各国消耗资源时对自然环境造成的冲击程度。阿拉伯联合酋长国是在地球上留下最大“生态脚印”的国家。据此完成下列各题。

设函数f（x）=$\left\{\begin{array}{c}{2}^x,x<0\\ {\log }_{2}x,x>0\end{array}\right.$，若对任意给定的t∈（1，+∞），都存在唯一的x∈R，满足f（f（x））=2at²+at，则正实数a的最小值是（　　）