—______. This is my friend.

某科学家在蚯蚓体腔液中发现一种能溶解细菌、酵母菌、原生动物及绵羊红细胞等的蛋白质CCF﹣1．结合免疫的相关知识，下列推论中正确的是（　　）

①B中可通过____判断可逆反应2NO₂⇌N₂O₄已经达到平衡．

②若平衡后在A容器中再充入0.5mol N₂O₄，则重新到达平衡后，平衡混合气中NO₂的体积分数____（填“变大”“变小”或“不变”）．

③若容器A中到达平衡所需时间t s，达到平衡后容器内压强为起始压强的0.8倍，则平均化学反应速率v（NO₂）等于____

④若打开K₂，平衡后B容器的体积缩至0.4a L，则打开K₂之前，气球B体积为____ L．

A man:Excuse me, how can I get to Beijing Station?

Franks:Go along this street. Turn right at the first crossing and Beijing Station is on your right.

A man:Wait a moment. Go along this street, and Beijing Station is on my right.

Franks:That's right.

A man: I want to go to the History Museum,too. Where is it?

Franks:It's on Renming Road. You can take bus No.1.

A man:Thank you very much.

Franks:Not at all.

在联欢会上，三（一）班有7名同学唱歌，有5名同学跳舞，其中有2人既唱歌又跳舞，还有另外25人表演了小品，三（一）班一共有多少人？（　　）