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    Everyone knows that exercise is important. We all need to exercise. Doctors say it's good for us. It keeps body strong.

    When the daytime comes, we must get up. This is the time for exercise. Exercise means doing things with the body. There're many ways to exercise. You can walk, run, swim, skate, or play ball games. Make sure you exercise in the following(下面的)ways: You have to like what you're doing. Exercise enough—but not too much. It's good to exercise twice each week. Thirty minutes each time is enough. Try all kinds of things until you find one, two or three sports that feel right for you.

    Exercising can be fun. Friends can exercise together at a fitness center, or they can play sports together. How do you exercise?

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英国伦敦唐宁街10号，传统上是第一财政大臣的官邸，后成为今日普遍认为的英国首相官邸。唐宁街10号的这一历史地位起始于(    )

如图所示，在竖直向下的匀强磁场中有两根竖直放置的平行粗糙导轨CD、EF，导轨上放有一金属棒MN．现从t=0时刻起，给金属棒通以图示方向的电流且电流强度与时间成正比，即I=kt，其中k为常量，金属棒与导轨始终垂直且接触良好．下列关于金属棒的速度v、加速度a随时间t变化的图象可能正确的是（   ）