A bottle weighs more after air is put in ,__B_____proves that air has weight
一个瓶在装入空气后更重了证明空气有重量的.
选B it作形式主语代指前半句话. 前半句是句完整的句子,不能用that which 因为句子不缺成分时不能用代词that/which 排除CD A选项we 和前半句的主语a bottle不符所以选B为什么不能选which形成非谓语