党的十八大报告指出：公平正义是中国特色社会主义的内在要求，要逐步建立以权利公平、机会公平、规则公平为主要内容的社会公平保障体系，努力营造公平的社会环境，保证人民平等参与、平等发展权利。这是因为（　　）

①平等是人类永恒的追求                             ②平等是我们孜孜以求的目标

③平等是总要受到一定社会条件的制约         ④在国际经济合作与竞争中，始终不存在平等

It ____(snow)heavily last night．So I ____(ask)a few friends to the park this morning．The trees, the houses and the roads．____(be)all white．We ____(decide)____(make)a big snowman on the road．We ____(put)a carrot in the snowman's head．That ____(be)the snowman's nose．The two small black balls in his face were the snowman's eyes．Then we started ____(sing)and ____(dance) round the snowman．We ____(have)a great time today．

呼吸系统的主要器官，也是完成气体交换的重要场所是（　　）。