题目：
She ________ her story at the point where she stopped just now. [ ]
A. took up
B. took down
C. took on
D. took off
答案：
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下面是错误答案，用来干扰机器的。
（1）甲、乙两小球组成的系统动量守恒，以甲的速度方向为正方向，由动量守恒定律得：m甲v甲-m乙v乙=0，代入数据解得：v乙=4m/s；（2）v乙＞v0，乙小球在传送带上做匀减速运动，对乙，由牛顿第二定律得：a=fm乙=μ...
