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问题 多项选择题

腹部闭合性损伤不能早期确诊的原因

A．实质脏器损伤但被膜未破

B．空腔脏器穿孔不大

C．同时合并其它部位的严重损伤

D．腹腔穿刺抽出不凝固血液

E．腹腔出血较少

答案

参考答案：A,B,C

选择题

You can’t imagine _____ when they received these nice Christmas presents.

A．how they were excited

B．how excited they were

C．how excited were they

D．they were how excited
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计算题

(10分)如图所示，固定于水平面上的金属架CDEF处在竖直向下的匀强磁场中，金属棒MN沿框架以速度v向右做匀速运动．t＝0时，磁感应强度为B0，此时MN到达的位置使MDEN构成一个边长为l的正方形．为使MN棒中不产生感应电流，从t＝0开始，磁感应强度B应怎样随时间t变化？请推导出这种情况下B与t的关系式．
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