A:婴儿出生
B:精子成熟
C:卵子成熟
D:受精卵
--- Is her sister a teacher or a student?
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一个物体沿直线运动,从t=0时刻开始,物体的 xt ﹣t的图象如图所示,图线与纵坐标轴的交点分别为0.5m/s和﹣1s,由此可知( )