如图所示，条形磁铁静止在水平桌面上，闭合铝环从条形磁铁的正上方附近由静止竖直下落至桌面．则在下落过程中（ ）
A．铝环中产生方向不变的感应电流
B．磁铁对桌面的压力始终大于其自身的重力
C．铝环所受安培力的方向先向上后向下
D．铝环的加速度小于或等于g
A、铝环在下落过程中，穿过的磁通量变化，从而产生感应电流，故A错误；
B、当铝环的磁通量变化时，产生感应电流，从而出现安培阻力，导致磁铁对桌面的压力大于重力．当处于条形磁铁的中间时，没有磁通量变化，则也没有感应电流时，没有安培力，所以对桌面的压力等于重力，故B错误；
C、当铝环磁通量变化，产生感应电流，安培力总是阻碍相对运动，所以安培力一直向上，故C错误；
D、由B选项可知，加速度小于或等于g，故D正确；
故选：D
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I would like to make a comment on the plastic bags which we use in our daily life. I thought that the 1 effect (影响) from all these bags on our environment was well -known and that most people know they should not keep 2 them but collect them in the dustbin ( 垃圾箱). However, one of the 3 near my home seems to use 4 of these bags than ever before. Red ones, white ones and blue ones-it 5 no difference. They don't 6 about the problems of environment. They still use plastic bags to wrap (包装) most of the things every day. When I went shopping the other day, I bought two things, and I 7 three bags to wrap 8 . Is it necessary 9 so many bags? Everyone should realize 10 important it is to clean our environment. Yours sincerely, Tony Wang |