下列物质含有氧分子的是 (    )

菱形的周长为20cm，两邻角的比为1：2，则较长的对角线长为（　　）.

已知$\left\{\begin{array}{c}x=2\\ y=1\end{array}\right.$是方程组$\left\{\begin{array}{c}ax+by=5\\ bx+ay=1\end{array}\right.$的解，则a+b的值是（　　）

                                                                                                            兰溪棹歌^①
                                                                                                               戴叔伦
                                                                                     凉月如眉挂柳湾，越中山色镜中看。
                                                                                     兰溪三日桃花雨，半夜鲤鱼来上滩。
                                                              【注释】①兰溪，在今浙江兰西县西南。棹（zhào）歌：船歌。