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问题 选择题

如图，一个球形物体O静止放在水平地面上，并与竖直墙相接触，A、B两点是球与墙和地面的接触点，则下列说法正确的是（　　）

A．物体受三个力，重力、B点的支持力、A点的弹力

B．物体受二个力，重力、B点的支持力

C．物体受三个力，重力、B点的支持力、物体对墙壁的压力

D．物体受三个力，重力、B点的支持力、物体对地面的压力

答案

正确的对物体进行受力分析：

A、物体静止在水平面上处于平衡状态，根据平衡条件，物体所受合力为零，水平方向物体如果受到A点的弹力，则物体不能保持水平方向平衡，故A错；

B、物体静止在水平面上处于平衡状态，根据平衡条件，物体所受合力为零，竖直方向受重力作用，B点受地面支持力，两个力是一对平衡力，故B正确；

C、物体对墙壁的压力不是物体受到的力，故C错；

D、物体对地面的压力不是物体受到的力，故D错．

故选B．

多项选择题

血钙降低见于（）

A.肾小管性酸中毒

B.甲旁亢

C.多发性骨髓瘤

D.阻塞性黄疸

E.急性坏死性胰腺炎
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单项选择题

Questions 26～30


It was books that first captured my imagination about faraway places. TV travelogues always seemed the poor relation to the classic written accounts, although of course the pictures were rather better. And then there was the issue of authenticity. All those pretentious theatrical types dying of thirst in the desert, as if we didn’t realize there was a camera crew on hand to cater for their every need. These days programme-makers know that the audience is more sophisticated and the presence of the camera is acknowledged. But can a journey with filming equipment ever be anything other than a cleverly constructed fiction
I recently got the chance to find out, when I was asked to present two one-hour programmes for an adventure travel series. The project was the brainchild of the production company Trans-Atlantic Films, which wanted the series presented by writers and adventurers, as well as TV professionals. My sole qualification was as a journalist specialising in "adventure" travel. However, I was thought to have "on-screen" potential.
The first programme was filmed in Costa Rica. Within 24 hours of my arrival, I realized that this was going to be very different from my usual "one man and his laptop" expeditions. For a start, there were five of us—director, cameraman, sound recordist, producer and presenter. And then there was the small matter of ￡100,000 worth of equipment. I soon realized that the director, Peter Macpherson, was a vastly experienced adventure film-maker. In his case, the term "adventure" meant precisely that. "Made a film with X," he would say (normally a famous mountaineer or skier), before describing a death-defying sequence at the top of a glacier in Alaska or hand-gliding off the Angel Falls in Venezuela. Invariably, these reminiscences would end with the words: "Had a great deal of respect for X. Dead now, sadly... "
Part of the brief for the series was to put the presenter in unusual situations and see how he or she coped. One such sequence was the night we spent in the rainforest canopy near the National Park in Guanacaste province. I don’t have a head for heights and would make a poor rock-climber, so my distress is real enough as the camera catches me dangling on a rope some 30 metres up, well short of the canopy platform.
Ironically, it was the presence of the camera, looking down on me from above, that gave me the impetus for the final push to the top. By this time, I’d learnt how "sequences" were cut together and realized that one last effort was required. I had to struggle to stay coherent while the camera swooped within a few millimeters of my face for my reaction In the end, it was a magical experience, hightened all the more by the sounds of the forest—a family of howler monkeys in a nearby tree, amplified through the sound recordist’s headphones.
Learning how to establish a rapport with the camera is vital and it took me a while to think of it as a friend rather than a judge and jury. The most intimidating moments were when Peter strolled up to me, saying that the light would only be right for another 10 minutes, and that he needed a "link" from one sequence to another. The brief was simple. It needed to be 30 seconds long, sum up my feelings, be informative, well-structured and, most important of all, riveting to watch "Ready to go in about five minutes" he would say breezily.

The writer uses the sequence filmed in the National Park as an example of ______.

A. something he had been worried about before any filming started
B. the sort of challenge that presenters were to face in the series
C. something he was expected to be unable to deal with
D. the technical difficulties involved in making films in certain places
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