【正确答案】
Dear Wang:
   Thanks too much for inviting US to join your son's birthday party. I have seen the lovely baby before and it is worth celebrating for him and your happy family.
   But I regret that I cannot go to the party at that time. You know our company has a branch in America and it has something serious happened which needs my immediate decision. Thus I should go back as soon as possible.
   I am really sorry for my absence. Hope you will have a wonderful party and a good time.
   

【答案解析】 此篇作文应为回复邀请的信函。在这类信函中要注意信的格式。时间应该写在右上角，称呼应顶格并首字母大写。正文要写清楚应邀还是拒绝，若拒绝，要写清楚理由，并向邀请人致歉。