12.BD【解析】在A与C发生相对滑动之前F=(mA+mn+mc)a=10t(N),解得a=2t(m/s2),加速度随着时间t增加。当A、C将要发生相对滑动时,对C进行受力分析可得pmAg=mcac,解得ac=4m/s2,此时t=2s,B正确;A、C发生相对滑动后,木板的加速度不变,A错误;发生相对滑动之后,根据牛顿第二定律可知F-mAg=(mA+mB)a2,Fr-mmAg=mAa2,得2+doFT: F2+2a加速度不断增加,比值不是定值C错误;A、C刚要发生相对滑动瞬间,长木板C加速度达到最大,最大加速度为am=2g=4m/s2,此时拉力F=(m+mB+mc)an,则F=20N,故历时2,由于A、B、C共速根据动量定理可得∑F,=(mmB+mc),求得v=4m/s,D正确。飞

二、读后续写范文(满分25分)Possible version 1Paragraph 1:It was time to put an end to the Magic Marty show. I walked over. "It was fishing line. " I sneeredfolding my arms. Marty subconsciously fingered the fishing line and nervously whispered, "So, are yougoing to tell people? Seeing the panic in his eyes, somehow, I hesitated. Should I_reveal the secret, Martymight lose his new_ friends. Finally, after struggling for a moment, I replied, "Nah, I won't say anythingMarty let out a relieved sigh, and I turned to walk awayParagraph 2Waitr"Marty jumped in front of me. Unexpectedly, he patted my arm, grinning. "You' ve got a prettygood eye for magic. Want to join me? "I nodded joy fully. That's how the Magic Marty and MysteriousMatt Lunch Show began. Now we've been a magic team for three months. And since I became a fellowmagician, Marty has let me in on some of his other tricks. It turns out that instead of a fake magician, Martyis a real friend.(150 words

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