根据提示写一封60-80词的信。
     假设你是Tom，Jim邀请你参加他这周日的生日宴会，但你的父母去上海要到下个月才回来，
奶奶又卧病在床，需要照顾，而且英语考试即将来临。因此，你无法前往参加生日宴会。于是，
你写信辞谢并表示歉意。信的开头和结尾已经给出。
Dear Jim,
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                                                                                                                                         Yours, 
                                                                                                                                          Tom