I have a friend. His name is Jack. ③He is healthy because(因为) he plays sports. He runs every day. He runs to his ___①____ in the morning and he runs ____②____ after school in the afternoon. He can run fast. He likes playing soccer, volleyball, tennis, basketball , ping-pong and more. He has three soccer balls, two volleyballs, six tennis rackets, four basketballs and many ping-pong balls. They are in his room. He plays sports with his friends every day. He likes many sports stars and he often watches them on TV. All my classmates say Jack is sporty (爱好体育)。

读课文《我们的知识是有限的》，填上关联词：

故此，____我不能准确地断定彗星的形成之因，____我是应当受到宽宥的，____我从未声言能够做到这一点，____我懂得它会以某种不同于我们任何臆度的方式形成。

2016年美国总统大选，花费近68亿美元。在这个4年一度的“砸钱游戏”中，由于美国大选的选举人团制度规则，能起到决定性作用的却并不是全国的选民们，而是少数富豪和财团。由此可见（    ）

①美国的民主具有真实性

②民主总是属于统治阶级

③国家民主都是相对的

④民主与专制总是统一的

一个平行板电容器，使它每板电荷量从Q₁＝3.0×10^－5 C增加到Q₂＝3.6×10^－5 C时，两板间的电势差从U₁＝10 V增加到U₂＝12 V。求：

①这个电容器的电容多大？

②如要使两极板电势差从U₁＝10 V降为U₂′＝6 V，则：每板需减少多少电荷量？

③若只把电容器极板间的距离减半，它的电容变为多大？