对以下实验装置所研究物理问题的描述，正确的是（）

如图所示，弹簧的上端固定在天花板上，下端连一小球A，球A与球B之间用线相连．A、B的质量均为m，弹簧与线的质量均可不计．开始时它们都处在静止状态．现将A、B间的线突然剪断，在剪断线的瞬间A、B的加速度的大小为（   ）

2016年6月6日至7日在北京举行了第八轮中美战略与经济对话。双方就重大双边、地区和全球性问题深入交换意见，中美双方强调全面履行在历年中美战略与经济对话中做出的承诺非常重要。双方表明将采取进一步具体措施支持强劲的国内和全球经济增长，促进开放的贸易与投资，强化和支持金融市场稳定与改革。下列对此认识错误的是（    ）

Teenagers growing pains

    Today, our students are facing many different problems and they want to ask some experts for help. Nancy says she feel stressed these days because of too much homework. She hasn't enough time to sleep for a long time and often falls asleep during the classes. John says he doesn't like to study because of the bad effect of computer games. Cindy says she gets fat because she likes to eat junk food and hardly ever takes exercise. Frank has the worst problem. He always fights with others as he doesn't know how to get on well with others and few of his classmates would like to talk with him.