一些政治团体或政党的出现，推动了历史的发展。

完全相同的三块木块，固定在水平面上，一颗子弹以速度v水平射入，子弹穿透第三块木块的速度恰好为零，设子弹在木块内做匀减速直线运动，则子弹先后射入三木块前的速度之比为____，穿过三木块所用的时间之比____．

A single-parent child should have the right to enjoy normal parental love from both just as his other peers, which some psychologists see as _____to their emotional well-being.