某女生了一对“龙凤胎”(一男一女),其原因是( )
A:一个卵细胞和两个精子结合
B:两个卵细胞和一个精子结合
C:两个卵细胞和两个精子结合
D:同一个受精卵分裂为两个独立的个体
如图所示为一皮带传动装置,右轮半径为r,a点在它的边缘上.左轮半径为2r,b点在它的边缘上.若在传动过程中,皮带不打滑,则a点与b点的向心加速度大小之比为( )
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阅读,判断句子正(T)误(F)。
Dalong is ten. He likes swimming. This is his father, He likes basketball. This is his mother. She likes table tennis and skiping. This is his sister, Lanlan, She likes skipping and swimming. She doesn't like football.