【正确答案】 C

【答案解析】[考点] it的用法
[解析] it在这里代表前面所提到的事实。故选C。注意区分A bottle weighs more after air is put in. It proves that air has weight. (两个句子)与A bottle weighs more after air is put in, which proves that air has weight(一个句子)。