a bottle weighs more after air is put in,__proves___air has weight

which proves that air has weight

which 指 前面的句子 a bottle weights more after air is put in. 冲入空气后的瓶子偏重

that 是指后面的从句 air has weight 空气有重量

整个句子的意思是。 冲入空气后的偏重证明了空气有重量。

which, that
用which是非限制性定语从句，指代前面整个句子，是瓶子装了空气后更重这个事实证明。。。用which指代前半个句子
而prove 加了that才能加句子

which,因为它是指代前半句话

which,that