Here are  a few sentences from the letter of thanks which Zhang Ying wrote to Professor Liang to express his gratitude, because Zhang Ying were recommended by Professor Liang to get further education in Sydney University last June, and he has been admitted to that university.   Please put the following sentences in the correct order.and choose which order is the correct one: ①　Dear Professor Liang,②　It was your very generous help that enabled me to obtain this splendid opportunity for further education. Thank you again and my best wishes to you.③　Last June, when I applied for the post graduate program in Sydney University, you gave me a lot of valuable help. ④　Yours truly,⑤　You not only wrote a recommendation letter for me to Professor Wells of the University, but also gave me careful and patient instructions on how to fill out the application forms and write the application letters.⑥　I am writing to express my gratitude to you, because with your help I am now a student in the Department of Chemistry at Sydney University.⑦　Zhang Ying