如图所示，长木板放置在水平面上，一小物块置于长木板的中央，长木板和物块的质量均为m，物块与木板间的动摩擦因数为μ，木板与水平面间动摩擦因数为 ，已知最大静摩擦力与滑动摩擦力大小相等，重力加速度g.现对物块施加一水平向右的拉力F，则木板加速度a大小是   (  )

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世界历史上，国家之间的结盟，不同利益集团间的较量，一定程度上影响着历史发展的进程，影响着世界格局的演进。阅读下列材料，回答问题：
材料一 

19世纪末20世界初欧洲的两大军事集团示意图
材料二  1942年1月，中、美、英、苏等26个国家在华盛顿共同签署了《联合国家宣言》。签字各国保证竭尽全力，互相合作，彻底打败德、意、日三国轴心及其附庸，决不单独与敌人停战或媾和。
材料三 

苏联领导人赫鲁晓夫和美国总统肯尼迪的较量（漫画）
材料四 

请回答：
（1）材料一图片中代表的军事集团名称分别是什么？ 两大军事集团的较量引发了什么世界性战争？这场战争结束后形成了怎样的世界政治格局？
（2）材料二中26国签署《联合国家宣言》的目的是什么？
（3）材料二、材料三反映了美苏间的关系发生了什么变化？这种变化形成了怎样的世界格局？
（4）据材料四的图片信息，“未来政治格局”呈现什么趋势？
（5）综合上述材料，分析指出影响国家间结盟与对抗的根本原因。

（2015·茂名）阅读下面文章，根据文章内容，从每小题所给出的A、 B、 C、 D四个选项中选出最佳答案。

As a young man, Tom was a famous artist with a wife and two fine sons. One night, his older son was ill. Tom and his wife thought it was nothing serious. But the boy died suddenly that night..

After his son died, Tom always felt very sad. To make matters worse, his wife also left him later, leaving him alone with his six-year-old younger son, Emie. Sadly, he turned to alcohol(酒) for help.

As time went by, Tom began to lose everything he had---his land, house, etc. A few months later, Tom passed away alone in a small bar. Hearing of Tom's death, I thought,“What a complete failure!”

But later, I began to change my earlier opinion. I knew Tom's now adult son, Emie. He is one of the kindest, most caring men. I saw the love between Emie and his children. And I thought that kindness and caring had to come from somewhere.

One day, I asked him what made him become such a specia1 person. Emie said quietly, “My father came into my room every night, give me a kiss and said,“love you, son.”

Hearing his words, I understood everything. Tom didn't leave many things behind. But he had been a kind loving father, and left behind his best love.