A.They were frightened and distressed. B.They swam away when the speaker was turned on. C.They swam closer to 'examine' the speaker when it was turned off. D.They didnt seem to be frightened and kept swimming near the speaker.
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【单选题】竖直向上抛出一个物体,由于受到空气阻力作用,物体落回抛出点的速率小于抛出时的速率。在这过程中( ) A .物体的机械能守恒 B .物体的机械能不守恒 C .物体上升时机械能减小,下降时机械能增大 D .物体上升时机械能增大,下降时机械能减小