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正确答案：

Dear Jack，
Thank you SO much for your warmhearted invitation of spending my summer vocation at your hometown，a quiet and beautiful country． I have been waiting for it SO long． But，unfortunately，I have to goback my own hometown，a hot and noisy city，for my sister just got an admission from a university and she
is preparing for it．As an elder brother，I must help her in both mental and physical ways So sorry for missing the good chance to join you at your home．Next year，we will spend summer together，our summer．
Send my greetings to your parents，please．I'll visit them next year.
Take care!
Li Yuan