如图所示，水平传输带以4m/s的速度匀速运动，传输带两端A、B间的距离为20m，将一质量为2kg的木块无初速地放在A端，木块与传输带间的动摩擦因数为µ=0.2．试求：木块从A端运动到B端所用的时间？

如图所示，质量为m的木块，被水平力F紧压在倾角为θ=60°的墙面上处于静止．则关于木块的受力情况、墙面对木块的作用力（压力与摩擦力的合力），下列说法正确的是（　　）

阅读理解

    Everyone loves Adventureland! The Parks and Exhibitions were built for you to explore(探索), enjoy, and admire their wonders. Every visit will be an unforgettable experience. You will go away enriched, longing to come back. What are you going to do this time?

The Travel Pavilion

    Explore places you have never been to before, and experience different ways of life.

    Visit the Amazon jungle(丛林) village, the Turkish market, the Tai floating market, the Berber mountain house and others. Talk to the people there who will tell you about their lives, and things they make. You can try making a carpet, making nets, fishing...

The Future Tower

    This exhibition shows how progress will touch our lives. It allows us to look into the future and explore the cities of the next century and the way we'll be living then. Spend some time in our space station and climb into our simulator(模拟装置) for the Journey to Mars!

The Nature Park

    This is not really one park but several.

    In the Safari Park you can drive among African animals in one of our Range Cruisers: see lions, giraffes, elephants in the wild. Move on to the Ocean Park to watch the dolphins and whales. And then there is still the Aviary to see...

The Pyramid

    This is the center of Adventureland. Run out of film, need some postcards and stamps? For all these things and many more, visit our underground shopping center. Come here for information and ideas too.

阅读理解

    Everyone loves Adventureland! The Parks and Exhibitions were built for you to explore(探索), enjoy, and admire their wonders. Every visit will be an unforgettable experience. You will go away enriched, longing to come back. What are you going to do this time?

The Travel Pavilion

    Explore places you have never been to before, and experience different ways of life.

    Visit the Amazon jungle(丛林) village, the Turkish market, the Tai floating market, the Berber mountain house and others. Talk to the people there who will tell you about their lives, and things they make. You can try making a carpet, making nets, fishing...

The Future Tower

    This exhibition shows how progress will touch our lives. It allows us to look into the future and explore the cities of the next century and the way we'll be living then. Spend some time in our space station and climb into our simulator(模拟装置) for the Journey to Mars!

The Nature Park

    This is not really one park but several.

    In the Safari Park you can drive among African animals in one of our Range Cruisers: see lions, giraffes, elephants in the wild. Move on to the Ocean Park to watch the dolphins and whales. And then there is still the Aviary to see...

The Pyramid

    This is the center of Adventureland. Run out of film, need some postcards and stamps? For all these things and many more, visit our underground shopping center. Come here for information and ideas too.

阅读理解

    Everyone loves Adventureland! The Parks and Exhibitions were built for you to explore(探索), enjoy, and admire their wonders. Every visit will be an unforgettable experience. You will go away enriched, longing to come back. What are you going to do this time?

The Travel Pavilion

    Explore places you have never been to before, and experience different ways of life.

    Visit the Amazon jungle(丛林) village, the Turkish market, the Tai floating market, the Berber mountain house and others. Talk to the people there who will tell you about their lives, and things they make. You can try making a carpet, making nets, fishing...

The Future Tower

    This exhibition shows how progress will touch our lives. It allows us to look into the future and explore the cities of the next century and the way we'll be living then. Spend some time in our space station and climb into our simulator(模拟装置) for the Journey to Mars!

The Nature Park

    This is not really one park but several.

    In the Safari Park you can drive among African animals in one of our Range Cruisers: see lions, giraffes, elephants in the wild. Move on to the Ocean Park to watch the dolphins and whales. And then there is still the Aviary to see...

The Pyramid

    This is the center of Adventureland. Run out of film, need some postcards and stamps? For all these things and many more, visit our underground shopping center. Come here for information and ideas too.

两个相同的小球A和B，质量均为m，用长度相同的两根细线把A、B两球悬挂在水平天花板上的同一点O，并用长度相同的细线连接A、B两小球，然后，用一水平方向的力F作用在小球A上，此时三根细线均处于直线状态，且OB细线恰好处于竖直方向，如图所示．如果不考虑小球的大小，两小球均处于静止状态，则

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

The main cause of death and disability in the US

    Stroke (中风) is the main cause of death and disability in the United States. Every 45 seconds, someone in America has a stroke. {#blank#}1{#/blank#} It is the nation's third leading cause of death, ranking behind diseases of the heart and all forms of cancer. The following is something more about stroke.

Definition and description of a stroke

    Stroke is a type of cardiovascular disease. It affects the arteries(动脉) leading to and within the brain. A stroke occurs when a blood vessel（血管） that carries oxygen and nutrients to the brain is either blocked by a clot or bursts. {#blank#}2{#/blank#} So stroke forms.

{#blank#}3{#/blank#}

    The American Stroke Association has identified several factors (因素) that increase the risk of stroke. The more risk factors a person has, the greater the chance that he or she will have a stroke. Some of these you can't control, such as increasing age, family health history, race, and previous stroke. {#blank#}4{#/blank#} Factors resulting from lifestyle or environment can be improved with a healthcare provider's help.

Breakthroughs in treatment

    {#blank#}5{#/blank#} In fact, about 600,000 people have strokes every year. For many years, there was no hope for those suffering a stroke. However, recent breakthroughs have led to new treatments. For the treatments to work, the person must get to a hospital immediately.

某同学在学校长为25m的泳池进行50m仰泳比赛中，若测得他（可视为质点）游完全程的时间为100s，则他在这50m仰泳中（　　）

海子爷与老雪狼

郭雪波

    ①漠北。苦寒之地，有一大泽，老百姓管它叫天海子。天海子西畔一隅，扎着一座地窖子，里边住着海子爷。

    ②可以这么说，天海子周边百里就剩海子爷这么一位两条腿的活物了。当初大迁徙，儿孙们跪着求他，一块儿走了吧。海子爷晃脑袋说，不。儿子说，这儿已没法活人了。海子爷说，我有法活，开春儿我就往海子边儿撒草籽儿插树条子。儿子没辙，留足过冬食物抹着泪一步三回头地走了。可没有熬到过年，一场沙尘暴就把他的房子卷翻了，老汉便挨着天海子挖了一个地窖子穴居起来，就如一只老狼苦守着这片被弃的土地。

    ③天海子边崖下有一洞穴，洞口上遮着沙蓬子。海子爷每天那经过都要吹吹口哨。那丛沙蓬子下便有了动静，若有若无的两点绿光十分微弱地在那里闪动。海子爷的嘴角呈现出微笑，心说：老伙计，还活着，活着就好。

    ④如往常一般，海子爷到天海子冰面，就往冰窟的深水放鱼钩鱼线。等着鱼儿咬钩。今天的头条鱼，半个时辰之后才上钩。海子爷从钩上取下那条鱼往身侧土筐里扔，习惯地回头望了望，想到不远处蹲着的老雪狼，兀自笑了。每当扔头条鱼时，他都会这样。

    ⑤那是三年前的事。当他把头条鱼扔进身侧土筐里，转瞬，鱼就被一只老狼叼走了。那老狼一瘸一瘸的，跑起路来歪歪扭扭，海子爷很快就追上了。他惊诧了：这老雪狼竟是他多年前的“冤家对头”——当年被人们围剿后逃脱的那只母狼。海子爷还清晰地记得，它和一只公狼被大伙围剿到天海子边，公狼为保护它而掉进天海子的那一刹那，它惨烈的嚎叫。想到这，海子爷盯视片刻，挥挥手说，你走吧!老雪狼低垂的雪色长尾冲海子爷摇了摇，咬着鱼蹒跚而去。海子爷一直目送着它回到天海子边沙崖下的那个洞穴，他终于相信了母狼一直守着天海子的传说。

    ⑥想到这些，海子爷便心生感叹。在冰天雪地的海子边，也只有他偏不肯逃离这片故土，还守在这里了。这些年，他们仿佛就成了一对彼此熟悉的伴儿守在这片荒漠。

    ⑦海子爷每天钓完鱼，路过老雪狼的岩洞，总要从筐里拣出一条鱼，扔过去。老雪狼待他走远，便从那丛沙蓬子后走出来，找到那条鱼，冲着海子爷的背影呜呜嚎两声，便转回穴内。今天也不例外。

    ⑧当日夜里，北风刮得紧。天海子边传来老雪狼的哀嚎。一早醒来，想到老雪狼昨夜的哀嚎，他放心不下要去看看。到了洞口，海子爷依旧吹起口哨，似有似无的绿点过了好久才出现，他松下心来。

    ⑨他下到天海子的冰面，凿了两个冰窟把鱼线放下去。可等了半天鱼就是不咬钩，海子爷冻得坐不住了，装备收线回家。突然，鱼线哧溜溜往水里窜，海子爷大喊一声“好大的鱼！”他拼命拽住线不松手。大鱼拼命地往水下逃窜。他猛地一个趔趄，便被那根鱼线倒拽进了冰窟里。海子爷从水下挣扎着冒出头，攀住冰窟边沿，攀爬，滑落，他的四肢开始变得僵直、麻木，海子爷有些无望了。突然，他的衣袖被什么紧紧地拽住了，抬头一看，竟是那只老雪狼。此时，老雪狼一双昏花模糊的老眼冒出很强的绿光，低着头，弓着腰身，撅着屁股，拼命拽拉渐渐下沉的海子爷。

    ⑩“谢谢你，老伙计！”海子爷冻紫的嘴巴张了张。

    ⑪唿儿——唿儿。老雪狼的喉咙里滚动有声，显然催促着海子爷赶紧使劲爬。可他麻木的四肢已经不太听使唤。老雪狼恼怒起来。呜呜低吼着，咆哮着，又拉又拽。老雪狼不走，也不松口，只一个姿势，低着头，弓着腰身，撅着屁股，拼命拽拉渐渐下沉的海子爷。慢慢地，它的身体变得僵硬，它的尖嘴自咬上海子爷的袖子就没有松开过。姿势也没有改变过。渐渐地，它的身躯连着海子爷一起被冻硬冻僵，纹丝不动了。

    ⑫海子爷和老雪狼就成为一对连体的活标本，仿佛是一对冰雕矗立在天海子空旷的冰面上，成为天海子的一部分，守护着这片天和地。