阅读诗歌回答后面小题。
                                                                                                              跌 倒
                                                                                                              牧 也
                                                                                                       风，跌倒了
                                                                                                      才有了美丽的落叶
                                                                                                      云，跌倒了
                                                                                                      才有了滋润大地的雨水
                                                                                                       所以
                                                                                                       我们不再害怕跌倒
                                                                                                       让我们在跌倒时
                                                                                                       用最美丽的姿势
                                                                                                        站起来

空气中含量多且化学性质不活泼的气体是（　　）