下列操作中，能鉴别空气、氧气和氮气三瓶气体的是        （   ）

已知两点F₁（-1,0）、F₂（1,0），且$\left|F_1{F}_2\right|$是$\left|P{F}_1\right|$与$\left|P{F}_2\right|$的等差中项，则动点P的轨迹方程是(      )

阅读理解

    Imagine you are in a jumping contest. Animals and insects can also enter this contest. But they might just leave humans in the dust!

    The first event is the long jump. The human athlete is Mike Powell. In 1991, he jumped nearly 30 feet, which is the world record for the long jump. That is about five times his own height.

    His competitor in the long-jump contest is a frog named “Rosie the Ribbiter.” Rosie set the world record for frogs in 1986 by jumping more than 21 feet. Her record has never been broken by any other frog. Compared to Mike Powell, Rosie's jump is not that amazing. But wait! Rosie is only about 10 inches long when her legs are stretched to their full length. She can jump more than 25 times her size. Rosie, the frog, wins the long-jump event.

    The next event is the high jump. Javier Sotomayor, the world record holder, can jump a bar (横杆) 8 feet high. That is about as high as the ceiling in most homes.

    Looking around for someone to challenge his record, Javier might need a magnifying glass (放大镜). The next contestant is a tiny insect called the spittlebug (吹沫虫).

    The spittlebug can jump 28 inches into the air. It is only a quarter-inch long, less than the width of a pencil. If the spittlebug were the same size as Javier, it would be able to jump 600 feet into the air. That is like a human jumping over a building 55 stories high!

    Though the humans lose the “jumping contest,” their competitors would probably agree that Mike and Javier are still pretty excellent athletes. If only Rosie and the spittlebug could speak!

阅读《蒲公英》，完成后面小题。
                                                                                                      蒲公英
        那是春日的一个上午，阳光明媚，清风徐来。草绿了，花开了，那些花儿，在远远近近的绿草间里像星一样闪烁。无数老人和孩子在草里流连花里漫步，也像春天的阳光一样灿烂。
        那时侯我正失意着，骑着车子在堤上乱走。一不留神，车子歪了下去，险些撞着坐在堤下的一个老人。在向老人表示了歉意之后，我没走，就坐在老人身边。
        那段日子，失意就像春天的草一样在我的思想里蓬蓬勃勃。很久以来，我看见一片落叶，便伤感，觉得自己也是一片落叶。我看见一片落花，也伤感，觉得自己是一片落花。看见流水，还是伤感，觉得自己的生命就在这平平淡淡中像水一样流逝了。
        老人看出了我的失意，跟我说起话来。老人说：“年轻人，怎么这样无精打采呢？”
        我手里缠着一根草，在老人问过后，我晃了晃那根草说：“我这辈子将像这根草一样平凡。”
        老人没做声，只是看着我。
        在老人的注视下我说了起来，我说我是一个很不幸的人，初中是因一场病休学了一年。此后，学习成绩一直很差，勉强读了高中后，又没考取大学。我又说一个人连大学都没有上过，毫无疑问是一个平凡的人，我这一辈子将在平凡中度过。我还说我是一个不甘平凡的人，我从小就立下志愿，一定要让自己的人生辉煌。
        说到这里我流泪了，我心里装不下太多的失意，它像汹涌的洪水，找到了决口。
        老人这时开口了，老人说：“你知道你手里是什么草吗？”
         “不知道。”
         “它是蒲公英。”
        “这就是蒲公英吗，我常在诗人的笔下见到它，可它也很普通呀。”我说。
        “你没看见它开着花吗？”
         “看见了，一种小花，毫不起眼。”
         “是不起眼，但它也可以辉煌。”
         “在诗人的笔下？”
         “不。”老人摇了摇头，注视着我。俄顷，他站了起来，跟我说：“我带你去看一个地方吧。”
         我听从了老人，也站了起来。
        随后，我跟着老人沿着那条堤往远处走，大约二十分钟后，我看见了一个足以让我这一生都震撼的景致。那是一块很大的河滩，有几十亩甚至上百亩大。整个河滩上全部是蒲公英，漫无边际。蒲公英开花了，那些毫不起眼的黄黄白白的小花，在阳光下泛着粼粼波光。那样美，那样灿烂，那样妖娆，那样蔚为壮观，炫目辉煌。
        一朵小花，也可以这么辉煌么？
        我们再没说话，就那样伫立着，起风了，花儿轻轻地向我涌来。我忽然觉醒了，心里一下子飘满了那些美丽的蒲公英。
         我觉得自己也是一朵蒲公英了！
         一直到现在，那漫无边际的蒲公英还在我眼里烂漫着，而且，在那里我看见了自己。