如图，一只钢球从圆槽的顶端滑下，若滑到最低点(O点)时，钢球受到的一切外力同时消失，则钢球的运动路线是(    ) 

-- Many students don't know what to           stress and become worried.

-- I think they'd better ask their teachers for help.

下列能够说明细胞核是遗传控制中心的一组是    (     )
①克隆羊多莉培育过程②“试管婴儿”的培育过程③转基因超级鼠实验④植物组织培养技术

$\frac{1}{3}+\frac{1}{2}$ =____

$\frac{3}{4}-\frac{5}{8}$ =____

$\frac{5}{9}\times \frac{3}{10}$ =____

$\frac{4}{7}÷\frac{2}{5}$ =____