材料：唐朝时期，书生张三出生在人杰地灵的A地（隋朝第二代皇帝三次乘龙舟来这里游玩，最后在这里被杀），张三苦读寒窗十载，一日他从家乡出发，经运河到京城应试。在离船登岸之地B(隋朝第二代皇帝曾在这里大兴土木，修建了豪华的宫殿)遇雨，全身棉布新衣湿透，避进一老翁家。老翁沏茶热情款待，为张三烘干衣服，还为他做了一顿香喷喷的占城米饭。张三又经长途跋涉，终于来到京城C，他看到全城街道整齐，树木成行，身着各式服装的边疆各族和各国侨民，云集到这里，往来的商旅熙熙攘攘，张三凭一手漂亮的楷书和一篇优美的文章及第（考中）。为答谢老翁，他送老翁越窑青瓷茶具一套、唐三彩骆驼载乐俑一个；还带老翁的儿子到京城C的“瓦子”中玩，在这里张三朗读了唐诗《忆昔》“忆昔开元全盛日，小邑犹藏万家室。稻米流脂粟米白，公私仓廪俱丰实”赢得了热烈的掌声。

下列命题中是真命题的是（     ）
①“若x²＋y²≠0，则x，y不全为零”的否命题  ②“正多边形都相似”的逆命题
③“若m>0，则x²＋x－m=0有实根”的逆否命题④“若x－$3^{\frac{1}{2}}$是有理数，则x是无理数”的逆否命题

根据材料内容，选择正确答案。

Dear Frank,    

    Thank you for your postcard. I'm very glad you are having a good trip now. Ottawa is very beautiful in May. I hope I will have a chance to go there one day.

    Our summer holidays will start next month. I enjoy my summer holidays, although(虽然)it's hot at that time in America. During summer holidays，I can swim in the sea and go fishing. It's so cool. And I am going to our hometown near Chicago with my sister Linda，and my parents agree(同意). Our uncle lives  there，so Linda and I can stay with him.

    We are not going to take a plane. We are going there by train. I am going to fish there. And Linda likes painting. She is going to do some painting there. I'm sure we'll have a good time.

                                                            Sincerely,

                                                              Jack