在同纬度地区，相对位置偏东的地点要比位置偏西的地点的地方时：

下列有关金属的说法正确的是（   ）

如图所示，置于竖直平面内的AB光滑杆，它是以初速为v₀，水平射程为s的平抛运动轨迹制成的，A端为抛出点，B端为落地点．现将一小球套于其上，由静止开始从轨道A端滑下，重力加速度为g．则下列说法正确的是（　　）

阅读理解

    It was a weeknight. We were a half dozen guys in our late teens, hanging around “the flat” — as usual. The flat was poorly furnished, which, after some time, grew tiresome. We wanted action.

    We got in a car, big enough for all of us to crowd into — and off we went, in search of adventure. We soon found ourselves at Blue Gum Corner, a place named after the huge old blue gum tree that stood by there, a well-known local landmark. It stands at a minor crossroad leading to our town. The trunk is tall and smooth with no handholds for climbing. About six metres from the ground the first branch sticks out over the road. We parked beneath the huge old tree and discussed what we might do. All of a sudden we hit upon an idea — a hanging! I was chosen as “hangee”.

    The plan was quite simple. As I stood upon the roof of the car, the rope (绳子) was threaded down my jacket through my collar and down one leg of my jeans. I put my foot through the loop at the bottom. There I hung, still. The boys rolled about laughing until, a car, I hear a car! Before they ran to hide in the nearby field, they gave me a good push so that “the body” would swing as the car drove by.

    To our disappointment, the car simply turned off for town without even slowing. The boys came out of their hiding places and we discussed the situation. Surely they had seen me, hadn't they? Then we heard another car. The act was repeated, but still without any obvious reaction. We played the game about five or six times, but as no one seemed to notice, we abandoned the trick.

    What we did not know was that every car that had passed had unquestionably seen “the body” and each one, too scared to stop, had driven directly to the local Police Station. Now at that time of the night, the local policeman was sound asleep in his bed, so the first person sent to the scene was the traffic officer that happened to be on duty that particular night. He had been informed that some person had been killed, by hanging, at Blue Gum Corner. When he arrived, the body was gone! And he was hearing “unnatural sounds” from the surrounding area. As far as he knew, some fierce animal was hiding in the field before him, possibly dragging a body behind him — and we thought we were scared!

    He went to his radio and made a call that really began to worry us. I lay so close that I could hear every word. He called for the “armed police” and a “dog team, better make it two” and he had a “serious situation” at Blue Gum Corner. Then the police officer arrived. After a briefing from the traffic officer he decided not to go into the field until armed police and dog teams arrived. Now two spotlights were on the field and none of us could move.

    As luck would have it, police cars cannot leave their spotlights on all night without charging their batteries. So, after a time, the two officers began lightening the field by turns, allowing us the opportunity to move on our fours for freedom. One by one, we all managed to steal away and make our way home. Behind us we left what must have looked like a small city of lights, police cars, roadblocks, barking dogs, armed officers and an old rope hanging from a tree.

    When I think back to that night, to that tree, to what the drivers of the cars think happened, to what police believe happened, and to what I know happened, I am reminded of a simple truth — our eyes see darkness and light, color and movement, our ears hear only vibrations (振动) in the air. It is how we explain these vivid pictures that shapes our “reality”.

2017年，在国家创新驱动战略下，北斗系统作为一项国家高科技工程，一个开放型的创新平台，1400多个北斗基站遍布全国，上万台套设备组成星地“一张网”，国内定位精度全部达到亚米级，部分地区达到分米级，最高精度甚至可以达到厘米或毫米级。最近北斗三号工程耗资9万元建成一小型设备，已知这台设备从启用的第一天起连续使用，第 $n$ 天的维修保养费为 $\frac{n}{2}+99.5\left(n\in N^{*}\right)$ 元，使用它直至“报废最合算”（所谓“报废最合算”是指使用这台仪器的平均每天耗资最少）为止，一共使用了多少天，平均每天耗资多少钱？