在图中画出磁感线大致的分布图。

植物开花、胃肠蠕动、萤火虫在夜间发光等所需能量直接来自

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    My father was always a good gardener. One of my earliest memories is standing without shoes in the freshly tilled soil, my hands blackened from digging in the ground.

    As a child, I loved following Dad around in the garden. I remember Dad pushing the tiller(耕作机) ahead in perfectly straight lines. Dad loved growing all sorts of things: yellow and green onions, watermelons almost as big as me, rows of yellow corn, and our favorite — red tomatoes.

    As I grew into a teenager, I didn't get so excited about gardening with Dad. Instead of magical land of possibility, it had turned into some kind of prison. As Dad grew older, his love for gardening never disappeared. After all the kids were grown and had started families of their own, Dad turned to gardening like never before. Even when he was diagnosed with cancer, he still took care of his garden.

    But then, the cancer, bit by bit, invaded his body. I had to do the things he used to do. What really convinced me that Dad was dying was the state of his garden that year. The rows and rows of multicolored vegetables were gone. Too tired to weed them, he simply let them be. He only planted tomatoes.

    For the first few years after he died, I couldn't even bear to look at anyone's garden without having strong memories pour over me like cold water from a bucket. Three years ago, I decided to plant my own garden and started out with just a few tomatoes. That morning, after breaking up a fair amount of soil, something caught the corner of my eye and I had to smile. It was my eight-year-old son Nathan, happily playing in the freshly tilled soil.

已知废旧普通干电池筒内主要有C、淀粉、MnO₂、ZnCl₂、NH₄C1等物质，某化学兴趣小组同学欲通过实验从中回收MnO₂，并研究MnO₂在分解过氧化氢制氧气的反应中所起的作用．

【查阅资料】淀粉不溶于冷水，完全燃烧生成二氧化碳和水；碳、二氧化锰不溶于水．ZnCl₂、NH₄Cl均易溶于水．