写一篇英语作文,根据提示写一封60~80词的信 （初二作文,快,来得及加10分,说到做到!）
汤姆邀请李萍参加他这周日的生日宴会,但李萍的父母出差要到下个月回来,奶奶又卧病在床,需要人照顾.而且英语考试即将来临,因此李萍无法前去参加生日宴会.她感到十分抱歉,只好写信辞谢.
Dear Tom,
Thank you very much for inviting me to your birthday party .I'd love to go but I can't___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
(快啊,今晚就要,来得及的加10分,最好自己写）