①如图（a），将轻质弹簧下端固定于铁架台，在上端的托盘中依次增加砝码，测得相应的弹簧长度，部分数据如下表：

②取下弹簧，用天平分别测出滑块A、B的质量（含挡光条）m_A、m_B；调整气垫导轨，使导轨处于水平，并调整两光电门的间距远大于弹簧的长度．

③在A和B间放入轻弹簧并将弹簧压缩，用电动卡销锁定，记录弹簧的压缩量x；静止放置在气垫导轨上；如图（b）

④释放弹簧，A、B滑块分别在气垫导轨上运动，读出滑块A、B的挡光条分别通过光电门的挡光时间t₁和t₂．

 材料：很多人看过那篇《妈妈爱吃鱼头》的文章，文章中的妈妈以“我爱吃鱼头”为由把鱼身上的肉都给孩子吃。我们也都有这样的体会：一道美味的食品，有人只尝一小口，就给另一个人吃；有人只吃到剩下一小口，才给另一个人吃——前者是母亲与孩子，后者是孩子与母亲。母亲只尝一小口，是为了知道食物是否烫嘴，是否变味，才放心让孩子吃。 

要坚定理想信念，坚守共产党人精神追求。对马克思主义的信仰，对社会主义和共产主义的信念，是共产党人的政治灵魂，是共产党人经受住任何考验的精神支柱。从生活与哲学上看，这是因为（  ）

    I was hurrying to our basketball game one morning when Hillary Barnes, the most popular girl in school, stopped me. "Are you the twin who won the art competition?" she asked.

    "No, that's Christine. I'm Katie." I said.

    We were not the same. In fact, we were quite dissimilar. She could beat Snow White in a beauty contest! Compared to Christine, I felt like one of Cinderella's ugly stepsisters.

    One Saturday Christine gave me a painting lesson because I was always trying to be artistic too. But I knocked over the vase of flowers. Instead of becoming angry, though, she turned the whole mess into a dreamy masterpiece*. You could look at it five different ways and see five different things. With this excellent creation, she won First Prize in the art competition. I really envied her, but I knew it wasn't her fault. I was the one with wild, unmanageable hair and no talent.

    Joining Christine's basketball team was my latest effort to be like her.

    On the way to the gym, I've made up my mind to give my best performance in that day's game. As soon as we arrived at the gym, I couldn't wait to warm up. "Christine, here!" I threw her a ball. It was too high and she missed it. The ball fell onto the recorder and broke it which was used to play the national anthem before each game.

    I apologized as I picked up the broken pieces of the recorder. Coach looked at the mess and asked, "How are we going to play the song now?"

    Christine suggested, "Let Katie sing it! She's always singing at home."

    Coach asked. "Would you?" I found myself nodding.

    Coach announced the national anthem and everyone stood. Standing before a microphone, I looked up to the flag. The notes seemed to flow from deep inside me, and my voice was clear. When I finished, the gym was silent. Then it was filled with applause. As the game started, teammates gave me the "thumbs up" sign.

    The next morning Hillary Barnes stopped me in the hallway and asked, "Are you the twin who sings?"

    "Yes, that's me." With smiles on my face, I walked to the classroom.